MIDI files are imported into OpenMPT's own MPTM format, but through the
Song Properties dialog you can convert the file to MOD format. However, as the MOD format is the least capable of all supported formats, you will have to keep in mind that this is a lossy conversion:
- Pitch slides will be inaccurate after conversion and need manual fixing
- Notes cannot overlap on the same channel (so there will be a hard cut if two notes follow each other immediately)
- If you use master volume, channel volume, etc., those will be lost in the MOD format
- It is advisable to replace the default imported MIDI instruments with single-sample instruments before conversion, because the MOD format does not support multi-sampled instruments. OpenMPT resolves all multisamples in the pattern data during conversion, which will make it more complicated to edit the MOD file afterwards. So if there are no multi-sampled instruments to begin with, the cleanup process afterwards will be simpler.
Note that you can also simply copy&paste between different songs in OpenMPT, so you can also create an empty MOD file, and copy the imported MIDI data from the MPTM file into it.