That would work but add the wood BEFORE you ever cut the existing rafter. Also use 1/2" OSB for your tie plates instead of the small metal plates that the gussets are normally put together with. Add the OSB on both sides of each rafter for rigidity. Also make sure the 2x's that you install are installed symmetrically. Post some pics when done. What you are wanting to do is no different than a factory made truss.