looking to add another bathroom upstairs but only the one soil stack the toilet would be on the same run as the present w/c is this possible

If it is possible to connect into the soil stack and is running/falling into the soil stack that's fine. Or you could get a saniflo installed this can be run on the toilet and connect the basin to it, then the outlet can be anything from 21.5mm pipe to 40mm pipe into the stack.

yes it is possible through a couple of different ways