Please double check this (there is a thread called "points that do/don't expire" in the Info Desk or MilesBuzz forum about point expiry and how to extend the life of your points) but I believe SPG counts points transfers from Marriott to SPG and back as qualifying activity (at least, that's what it shows on my account). But yes, if you transfer your Marriott points into SPG points and have a qualifying SPG stay, you should absolutely be good - these points are then basically SPG points and their expiration is dictated by SPG rules. (you can then likely transfer them back into Marriott if you'd like).