You have several ways to add warmth. For starters, look to your sleeping pad. If you're using a self-inflating pad, then consider adding another layer with a closed-cell foam pad. That would be something like a Cascade Designs Z-Rest ($25), which adds quite a bit of thermal insulation but only one pound of weight. Next, add a lightweight silk liner such as Design Salt's MummyLiner ($60). That should add anywhere from five to ten degrees to your personal comfort range.
After that it gets complicated. You could add a fleece bag liner, which would be warmer than silk but very bulky. Or you could simply stuff another sleeping bag into the Marmot. Sierra Designs makes what it calls a Knap Sack ($189), which is a very simple semi-rectangular sleeping bag. It would take your Massif to the below-zero level, but the price would be the cost of an extra bag and nearly three pounds of weight, probably defeating the purpose in getting the Massif. Try the extra pad and the silk liner and see if that does the trick. Last but not least, wear your spare clothes when you go to bed!