Personally, (and as a female, which might or might not be important 😕) I'd try to somehow 'initiate' a tickle fight in some sense. She says a smart remark, or you really see any reason to bug her over something she says - or does not say, for that matter - after gauging her reaction (how ticklish is she? Does she seem to have a stubborn streak, and try to get you back? Or does she seem to crumple like a deck of cards?), I'd try to then ease into mentioning that you have a greater 'interest' in tickling... a fetish. Then, depending on where the two of you have ended up, you might end up doting on her foot, mentioning your foot fetish, as well.
Either way, I'd try to create a situation where the mention comes up pretty soon, if you think there's any chance of developing a further relationship, as having things like this hidden... you just have to almost make her prepared for something 'worse', before you mention something "childish" like tickling's a turn-on for you.
I say, if someone told me they had a foot fetish, which would "normally" put me super-off, if a side-effect of that 'fetish' is offering as many foot massages as desired, that right there might make me change my mind (before I was able to come to terms with this particular interest of mine). If your girl likes her feet being massaged, I'd try to capitalize on that fact, if she seems averse to your having a foot fetish. Either way, you get more feet (and tickles), right? 😉
All the best either way, and PLEASE let us know how things go!