The Arduino USB host shield utilises Maxim's MAX3421E Usb controller. From my understanding of your post, You have three options here:
1) create one custom board in Arduino Uno form factor containing the RFduino module so it can control and carry the usb host shield.
2) create one custom board in any shape you need and integrate both, the RFduino module, the MAX3421E controller and all other supporting components.
3) create kind of an adapter cable from your RFduino connector to the usb host shield connector (supposing you're using some other RFduino shields too).
The second one seems best for me.