No harm in testing it.
Chances are one or the other is hugely louder a signal to the amp, since you can't mix them. With luck it's gonna be in the ballpark. Naturally the mic sound is gonna be colored hugely but it won't hurt to try. The only thing that could happen is you might be able to overdrive the amp input too much with the mic signal, but you'll know from the distortion level if it's too much well before anything breaks. And might be prone to feeding back like a mutha if played any louder.
I've seen a street musician play an acoustic with a magnetic pickup and a vocal mic both patched to a battery-driven Pignose, and the result was very much old blues indeed, kinda like listening to a gramophone recording, but it worked well for blues anyway.