I've been looking for ways to export a particular instrument from a file as a soundfont, but to no avail so far. Is this possible, and if yes, how?
No, this is not possible. OpenMPT is not a soundfont authoring tool. If it is absolutely necessary for you to have a DLS or SF2 file in the end, you can try saving your instruments in ITI or SFZ format and then convert them using Awave (https://www.fmjsoft.com/awavestudio.html#main).