There's a couple of options for you:
- hUGETracker can convert .mod to .uge, but it needs manual tweaking sometimes so it's not a perfect system
- Beatscribe has a tool for midi to uge, though you might need to download another program like OpenMPT to export a mod file as a midi file (https://beatscribe.itch.io/midi2uge-hugetracker-midi-convertor)
- For my tracks, I've already done the conversion to uge (https://tiptoptomcat.itch.io/8-bit-gameboy-songs-dx-gb-studio)
- You can try searching the GBStudio Discord/Reddit pages for more tips