Yes to use for example Modular V3 that's an instrument as an FX, then you route audio thru it and play the keys like megamarkd say.
The question is how you route audio thru it.
I don't know how you can do it in standalone mode.
Some DAWs like Studio One still needs a FX version of a synth to do it, and such a FX version unfortunate don't exist anymore as they did'nt in previous versions. Arturia should make one so all could have this functionality. It's a downgrade from earlier versions. If you have Modular V2, then you also have it in a FX version, and then you need to have it on a track so you can play it using MIDI notes to let the incomming audio from another track thru. How to do this also depend on your DAW.
Some DAWs can somehow route audio from one instrument on a track to another. In that case you can use the modular V3 instrument version. How it's done or if it can be done depend on your DAW.
In Modular you also have envelope followers. It's not just filtering you can do. Experiment.
Also be sure you use the audio in connections inside Modular in a preset. Otherwise modular does'nt use the audio for anything.
Arturias new software effects (filters and preamps) that's not instruments, come as FX versions and can just be added like any other effect in a DAW.
Good luck.