i dont think so, not on blackberrys. besides, SMS sigs are annoying and are always long and contain stupid designs. im glad that most people dont use them.
I just use the built in auto text for signatures. I have 4 email accounts connected to my Blackberry so I have a couple of different signatures. I have my business one set up so when I type "sig" and hit space it puts in my full business signature, but if I type "famsig" and hit space it puts in my personal signature. Hope this helps.