Your goal of having a 2-for-1 deal can be done. Stuff like this is being done all the time, but in other areas. Let me describe how this model works.
I work in Open Source Software. There are many people who volunteer their time to work on various projects. They do this because the like doing it. They volunteer their time, and contribute the source code that they write to projects.
Many of these software engineers get to be really familiar with the Open Source Software project that they are working on. They become experts in it. In many cases, companies will hire them, specifically to continue to work on the same Open Source Software projects, that these companies are then leveraging. These engineers are then doing work-for-hire, on stuff that they used to just do volunteer work on.
I hope you can see how this model can work for genealogy and WikiTree.
You are asking for someone who knows how to do professional-level genealogical work, but their output is put here on WikiTree. These two goals do not conflict, as others suggest. You would be paying said genealogist for their time and research capabilities, not for some proprietary output report. That is a different approach. Any professional genealogist who is not ok with having their output put here on WikiTree should be avoided.
By the same token, perhaps another route is a volunteer-going-pro. It might be better to seek to pay someone who is already a volunteer here, who knows how WikitTree works, but is skilled enough to go to the "next level".
I strongly disagree that these have to be separate roles. There are whole sections of the economy who are already used to a collaborative environment for their work, both volunteer-based that turns into pay-based.
Remember, it would be about paying for their skills and time as a service, not for some proprietary report.