System families in formulas?

I am building a window family and I would like to use the system families of Sill Height and Head Height in formulas (for example to calculate the Rough Opening Height -- Sill Height + Height = RO Height). But those parameters don't exist in the family, only in the project. Is there a way to do this?

Thanks in advance!

That doesn't make any sense. Sill Height is from floor level to underside of origin level in the family so it's often a value of 900-1200mm (3' - 4'). You need to add a parameter of 'rough opening offset' which should be 10mm (3/8") or so and then use that to offset the opening cut in the family from the outside of the frame of the window.
Then you would calculate your rough opening by adding width + (2 x rough opening offset) and then do the same for height.