Since universities do not normally pay the salary of a research professor, and salary along with research funding comes from external resources, this not a paid position. On the other hand, a few people are interested in a job without a secured salary (or a few researchers are confident enough to get enough funding to secure their salary too).
As a result, universities do not advertise for hiring
research professors (at least, I do not see in job websites). Therefore, it should be based on private negotiation. Logically, universities should not have limitation for the number of research professors, as they do not pay their salaries. If they can obtain fund, it is good for them, if not, the university loses nothing.
In the aforementioned circumstances, I guess, potential researchers should start this negotiation. If yes, how this negotiation is normally started? A potential research professor contacts the university President, Vice President for Research, Dean, Department Chair with a proposal?
What is the procedure for hiring a research professor?