yes! i love doing this, in fact i am building my eurorack partially around this concept. some synths have a built in gain stage for processing audio and bringing it up to synth level but many do not so there are a couple of ways of going about this:
1. some kind of pedal such as a boredbrain interfx, the interfx has 3 send return loops intended to be used to process modular signal with guitar pedals and then return it to the synth. basically it brings synth signal levels down to instrument level and then brings them back up to synth level to be placed back inside your system. it also has a switch to bring instrument level devices (guitar pedals contact mics the usual) into synths to be processed by the synthesizer. you have up to 3 different inputs available for this, which means 3 different pedal chains to be used as sources. it has a switch to switch back and forth between the two modes so you could have two inputs dedicated to contact mics or a vocal mic and one send return loop for running a synth through pedals etc. also, if you flip the switch the wrong way it drops it into a feedback loop. and if nothing is plugged in it also works as a VCA. so this would be the way to go if you dont have a eurorack system and just have desktop modular synths or if you want to save space in your rack. i have one of these and love it.
the second way is some kind of eurorack module, boredbrain also makes some of these but you will need several to replicate the interfx if that is your goal. instead of those i have a ears by mutable instruments and a byxa by slyph modular which is a clone of ears but with the addition of switches on the front for controlling how fast the gate envelope etc is. they have one 3.5mm input for instrument level (or line level! whatever.) devices, so if you had a mic or contact mic you'd plug that in there probably by using a 1/4 mono ts to 3.5mm mono ts adapter cable. the ears/byxa generates envelope and gate signals as well as a louder synth level version of the original signal to be used with your synthesizer. so you could, for example, run a contact mic or pedal feedback loop into the module and then feed that into whatever eurorack modules you want as well as use the envelope or gate to control a synth parameter such as a VCA or oscillator 1v octave input.. really whatever you can think of.
heres a
link to my eurorack setup so you can see what modules i use.
i'd been lurking this forum for quite a while but i saw this subject and thought to myself "hey! i know about that!" and signed up. i hope this post is informative :)