Or would you just have to ask your recommenders to re-send their letters again if you were to re-apply? I tried looking into this, but I wasn't able to find much about it.
I don't believe so. Optomcas gives you up to 4 spots for letters. I don't think you can use it otherwise applicants would be able to submit more LOR's on optomcas. I would contact optomcas to double check and make sure, but I don't think you can use inter folio. I think you would have to just ask your letter writers to resend their letter to optomcas.