I don't think you can port out of a wireless service without canceling it. Are you looking to no longer use that wireless phone? Do you still need the phone but want it to have a different number?

If the former, I'd sign up with a new number and set call forwarding from the wireless phone. If the latter, I do not have a simple solution in mind. The best thing I can suggest (complicated and VERY unsure f it would work) for that situation is to contact the wireless carrier and see if you can sign up with a new line OFF CONTRACT and see if the carrier would be willing and able to switch the contract from the number you want to port to the new line.

How long is remaining on the contract?