Can i choose another merchant account? I already have one set up to work with online payments so I don't want to apply for TransFirst.

I have an existing payment provider set up for my store that I recently set up to include online payment options. Since I have already paid to set this up, I would like to add this to my website. I do not want to apply for TransFirst as my merchant provider. How do I set up my existing provider with my SimpleStore? Thanks.