Your ManiaPlanet.exe is located in the Steam folder. Get to it by going to Steam/steamapps/common/ManiaPlanet_TMStadium
Alternatively, you can still download the non-Steam version of ManiaPlanet and use it (your ManiaPlanet account is independent of Steam). This also lets you use multi-environment titlepacks, and you can even link the .exe to Steam as a non-Steam game if you want the overlay.
As for converting TMUF/TMNF tracks to TM2, I've never done that so I can't be much help there.