IANAL and I'm too lazy to research this, but I do not believe you can add it. It was always OK to add a folder to a pumpgun even during the AWB, but I don't believe the same is true with semi autos as they don't get that exemption.
But someone will be along shortly to explain the real answer.
Edit: Just read the text of HR 1022 which is very similar to the original AWB and it has nothing that would prohibit this. I do believe MA's AWB was a carbon copy of the original Federal AWB.
That said, wait for more responses.