I want to try to develop some Pan F 50 I have problems with, replacing powder developer with liquid one, but the only liquid developer I have is from the tetenal C41 kit.
Well I just tried it myself. I just treated it like a regular developer, 9min @20ºC, stop and fix. Thinnest negatives ever, I can barely make out anything, but I might be able to see if the dark spots I'm getting with d76 are present here.
You'd have to greatly increase the development time because it's balanced for 37ºC which is why your negatives are thin, if you added colour couplers then if would have added to the silver image and be like XP2 (unbleached)
On a serious note I have developed B&W papers in Colour developer with specific colour couplers added, Tetenal made a kit to do this although I made my own up from raw chemicals. The first kit for this was Johnson Colourform - the link is to a leaflet I own.
In theory you could achieve very fine grain results using C41 developer and a colour coupler, that's all XP2 is except the coupler is already in the emulsion.