OpenMPT can do the trick.
All you need to do is delete all patterns after the loop, then insert a Position Jump command on the last note in the last pattern. If you need to jump to a certain note within a pattern, rather than the start of it, also put a Pattern Break command on the same line (you can only have one command per instrument, but you can just put it on a different instrument - it makes no difference which one it's on).
For the record, the same process can be used to convert MIDIs to ITs - just open them in OpenMPT, set up the looping, and do "Compatibility Export". I recommend using a proper MIDI editor to change their tempo to 128 first, then changing it back (in the patterns, not the global tempo) after opening in MPT. I have no idea why this works, but in my experience, it definitely does make a huge difference.