yes... almost anything "can" be a modulation source...
usually it helps if your modulation source is slower than what you are modulating...
all signals in modular are just electricity varying in frequency, amplitude band shape... so to a large extent anything can be anything...
the only real caveats are that for a signal to be audible it needs to be both fast enough (>20Hz ish) and slow enough (
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities