There are a few ways to go about this including probably a LOT that I am unaware of.
For me the easiest way if you are using a named subset in the active form is to put an extra row in (that is not part of the active form so must reference the same TM1RPTView Statement) and uses the Subset Name as if it were an element name - this then automatically consolidates the data and still performs as if it were a standard TM1 Cube Cell where you can spread over it etc etc etc.
If you are not using a named subset I tend use an excel sum formula (you need to set the range to be below the active form range so that it will expand with it - plenty of other posts on here explaining how/why on that one.)
This will however not allow you to interact with it as per a TM1 cube cell.