15-sample MODs are not just 31-sample MODs with less samples. They are an entirely different beast (with a similar file strucutre) and there is no practical reason for this feature since
1) Noone would practically use a 15-sample replayer routine for anything and
2) There is not just one 15-sample MOD format but a handful of them and OpenMPT already tries its best to guess which one of them was used. Introducing more ways to save such files will destroy any heuristics that OpenMPT and other players use to find out how to play these files. I really don't want to add any more mess to this table: http://www.un4seen.com/forum/?topic=14471.msg100829#msg100829