I have cross-section, and from it i can get particles momentum direction and momentum. For example i can generate file with data for number of primaries i need. For example (part1 E=100 cosx = 1 cosy = 0 cosz = 0). Can i put this file inside Fluka source defining routines?
yes, it is possible. You may have a look at the source_newgen.f user routine, especially it’s external phase space file. (13.2.20} source.f: user-written source | FLUKA, bottom half of the page)
However, since you are planning to generate the input file, it would be much easier to implement the sampling directly inside the source routine.