The trusses probably span a great open space down stairs, Unless you can find bearing walls and or beams with bearing all the way down to the foundation, I don't think you will be able to do it with out removing and replacing trusses. thw first picture in craigs post is what your after but in hand framed roof you have to support the floor at each end plus the new pony walls to support the roof mid span. I think the 1 floor is figured at about 100 lbs per sq ft and now your adding some portion of the roof load. the engineer is the only answer like was suggested.
The hole house was engineered for trusses and would have been alot more money to build a hand framed roof.