If the Add2psu is a gimick what's the best option to trip it on? 
A paperclip, metal staple, short length of wire etc....
You only need to connect the green wire to one of the black ones on the 24 pin plug.
^ this
Or, if you really want to b "neat," you could just use a 2-1 splitter for the ATX connector, which is ~$10. But a paper clip or short length of solid wire is really your cheapest alternative.