What do you mean by "fix an interest-only tracker"?
1. If you mean fix to the actual interest rate you are on now then no because the bank won't fix at such a low rate
2. Change to a fixed-rate from your tracker rate then yes the bank will probably take your hand off if you offer to give up your tracker. Even those banks who aren't offering fixed rates any more will probably offer you a fixed rate
Don't give up your tracker!