I don't know a thing about the "scenery"-fixed thermals, but I imagine you can put them all in the same folder, on top of the local areas sceneries.
However, FSX can generate its own thermals, depending where you are, so the easiest solution would be to turn on the thermal visualization option in the sim settings, and choose the "schematic" choice. This will show the thermals by drawing big 3D green spirals. The "natural" option will show eagles soaring in the thermals, much more realistic but also much harder to spot.
Also, some specific programs can be used to create ridge lift:
- CumulusX, freeware
- Active Sky (next or AS16), payware.
I used CumulusX for several years, then switched to ActiveSkyNext.