Hello Jos,
Unfortunately, the both are not supported yet.
For the guard expression, we will implement it in the next minor release (e.g. v2.4.0). Recommend to use Text annotation temporally.
But, the sub-numbering is not quite easy. If you don't mind about strict modeling, turn-off showing numbering (select the diagram in Model Explorer and uncheck 'showSequenceNumber' property) and then type the sub-number with the name.
Thanks,
StarUML Team
2015년 6월 17일 수요일 오후 9시 56분 51초 UTC+9, Jos van Weert 님의 말: