Just coming back to verify - the 9mm "ankle" (foot holder) and 1/4" foot from my Elna (Janome 9900 twin) fit my 7700 and will work with it.
The part at the bottom of the 9mm foot holder that grips the feet is wider than that on the one for my 7700, which is why it won't work with the 7mm feet that are standard with the 7700.