Dear @ArghyaC,
Similarly as it was discussed in Multiple complex source in Fluka, you can use a random number to select between regions.
Please note, the regionsource() subroutine is not part of the distributed version of the source_newgen.f user routine.
Also, since it is using a rejection method, the speed of the sampling is worse, than sampling directly from a cylinder and then roto-translating the coordinates to match the geometry.
Cheers,
David