if you leave yourself a few minutes at the end you can export your project to wav and then convert that to mp3 (i use a very simple program called LAMEdrop
to do this in seconds).
alternatively if your track loops: famitracker has a “play track x
times” field when exporting to wav. then drop that wav into audacity and put a fade-out on it then export that
to mp3. that’s what i did for this
since Dn-Famitracker is a fork of famitracker i’m assuming it retains these features.