In case the COM header can't be used instead, which I have doubts it can, but I'm not sufficiently sure that I dare state it as a fact, there are internal USB 9-pin splitters and expansion cards available - like
THIS ONE for example.
It is not a specific recomendation, it is just to show one way of being able to use a proper internal USB 9-pin connector when all the onboard ports are already occupied.
There are different variations available, some more expensive than others, but also with added features and functionality which could possibly be useful to some people.
EDIT : tried a different link to same product, I hope it actually works this time