mrrsm

I am attempting to add another delivery method for a different fax provider (MetroFax). I currently have it setup as a drop-in replacement of the interfax method but I feel like to properly integrate it I have to edit a lot of the "core" files so I could (in theory) use either interfax or metrofax. Is there a way for me to add another fax method without having to modify the core files which could break on an upgrade?