Yes, it is possible. Let's take a hypothetical situation. Let's suppose that I want to pull your credit report or put either an adverse or a positive report on your credit. I can do that if I join the credit bureaus and pass their membership requisites which are at least close to the following.
(1.) I have a store or an office in some office building.
(2.) I have a business telephone with a yellow pages listing.
(3.) I will submit at least 500 or more reports a month.
There may be other requirements that I'm not aware of however.

If I can't meet those requirements I can contract with someone else who can meet those requirements to allow me to piggyback my reports onto theirs. That way I can submit however many I want until such a time as I can meet the credit bureau requirements.
Theoretically speaking, I can set up a corporation and sell you a plan to send 12 or more reports to the credit bureaus, one a month, claiming that you are paying a loan I made you of whatever amount you want me to report. Say $100,000 for instance and that your payments are always on time and as agreed. You will pay me say maybe $300 a month to do that for you.

There have been many companies who have set up corporations to do just that and have succeeded in doing so. One of the most notable who at least claimed to be able to do that sort of advertised here for a short while was a company set up by Brandon Callier and Craig Cunningham of El Paso, Texas. They were also selling Authorized User trade line accounts back then.

They had formed a New Mexico non-profit LLC and were doing business out of El Paso, TX without having registered their LLC with the Texas Secretary of State. They also had not registered with the Texas State Attorney General's office as a credit repair organization and were operating out of Callier's home without having a City of El Paso business license.
Callier-Cunningham sued me for over a million dollars and among their ludicrous claims they said I had stalked Callier's wife and damaged their business reputation. Of course I had not done any stalking of anybody and even if I had they still could not bring that charge because only a U.S. District Attorney could bring such a criminal charge. The Texas State AG apparently agreed with me and their illegal credit repair scam was quickly put out of business and their web site suddenly disappeared from the internet about the same time. Needless to say, Judge Cardone also put their silly federal case out of business too once she learned the facts. (LOL).

Craig Cunningham recently got a big write up in the Dallas Observer telling how he now lives in a ramshackle house in Dallas and makes a killing filing lawsuits against debt collectors. Problem with that is that a Pacer search soon reveals that about 35 cases have been filed in Texas federal courts by persons named Craig Cunningham but only about half could possibly belong to this Craig Cunningham and there are other people in Texas named Craig Cunningham. That fits with the Observer story saying about 18 cases have been filed by the Craig Cunningham of the story. I've downloaded the summaries of all of them and it appears that he might have gotten a settlement in one case but lost all the rest. Of course, he usually attempts to file in Forma Pauperous so he don't have to pay filing fees. Quite a character to say the least.
Somehow another fellow named Steven Katz also got his picture prominently displayed just below that of Cunningham. Katz is denying any association with Cunningham but with a photo of him sitting behind a desk in a room filled with books and junk of various kinds including a radio of some kind that appears to have fallen off the overcrowded desk and hanging there by it's cord it is indeed hard to imagine how such a photograph would appear in a public newspaper if Katz had not agreed to its publication and most likely signed a press release in order for it to be there. If Katz had not posed for the picture and agreed to its publication he could easily sue the paper and win a huge damage award because upon publication in an article such as the Cunningham article the viewing public would automatically assume that the two must be closely associated.

Birds of a feather flock together type of association.
There have been some comments on both the ACA board (insidearm) and on Katz's board about the article but Katz isn't having much to say about it except one or two general denials of wanting anything to do with it or even comment on it.
Given Cunningham's history I wouldn't want to have shown up even remotely associated with him in any such newspaper story either.

But the answer to your question is that yes, anybody can put just about anything they want on anyone's credit reports if they want to without any proof whatever. They can do it by setting up their own accounts with the credit bureaus or they can do it by piggybacking with some shady debt collector who has an account with the credit bureaus and is willing to accommodate them for a small price. Doing that benefits the debt collector because it helps them maintain their quota of reports which they might not be able to maintain without selling to anyone who wants to piggyback with them for whatever reason, good or bad.

When it comes to the relationship between debt collectors and the credit bureaus we all know that one scumbag will lie and the other one will swear that it is the truth.

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