Is there such a thing?

i mean like an effects pedal that you could plug a mic and your guitar into, and it would mix the two to get the talkbox effect.

any ideas?
I don't think so, I've never heard of anything like that.
Main Gear:

ESP Eclipse CTM I
Dean 'Shards' Razorback
Chapman Guitars ML-1
Ibanez AEL40SE Acoustic
Blackstar HT-5 Mini Stack
EHX Metal Muff
EHX Small Clone
The very nature of a talkbox is that the sound wave being blasted through the tube is being shaped by you moving your mouth, then amplified through a PA.

Just singing into a microphone is not the same, so even if you could process the signal from a microphone and convert it into some kind of filter for the guitar signal, and plonk it all inside a stompbox, it wouldn't have the same effect.
no. The closest thing to what you're talking about is a wah pedal.

...or a vocoder ?
alright thanks guys, just tough it might be possible but i guess it just wouldnt be the same.