Can my fiance pay me 1/2 of the existing equity in my house and then be added to title by quit claim deed?

You should check with the mortgage holder, before you do anything, and get their approval in writing;  if your fiancee's credit is bad, there might be a problem.  Simply doing the quitclaim, without the mortgage company's knowledge or permission, might also create problems once that comes to light, because by giving her an ownership interest, you're damaging the bank's security interest.

I'd also suggest that this might not be something you want to do right away.  I know that you are going into this marriage expecting it to last forever, but it doesn't always work out this way.  Once you give her the legal interest in the property, there's a good chance you won't get that all back in a divorce.