What temperature ranges are you typically camping in?
I'd personally suggest that you send an email to the various vendors that we have here on HF, and ask them if they have anything that would meet your needs for covering both an HH Expedition and the WBBB.
You might also consider making yourself one from a poncho liner, if the temps you're sleeping in aren't so extreme.
Personally...I'd change out the mummy bag as soon as possible too. Either pick up a top quilt, or a larger, roomier sleeping bag. I have a 'standard' 20 deg bag my wife bought me and it's ok...but I'm more comfortable in my military issue ICW sleeping bag...much roomier.