There definitely isn't an additional open M.2 slot for WWAN, and unfortunately I don't know of any single cards that combine WiFi, BT, and WWAN. I think Intel has some cards that do WiFi, BT, and WiMAX, but that standard never really got off the ground and is not very widely supported by carriers. Sorry!
EDIT: And yeah, totally forgot about the lack of an antenna being an issue. Oops!