yea if you go to auto text and create a new one (bb menu > new) and make one for say "sig" and make it replaced with <your sig> you can just type sig at the end of the sms and it will auto append it. Closest you're going to get.
PIN is a unique alphanumeric number to your BB, you can find it under Options > Status or if you type "mypin" (no quotes)into any message it will insert it for you.
Typical use is for Blackberry Messenger. Exchange PIN's with each other, add to BBM and you have IM's between phones. Does have some other uses as well, but that's probably the most common