No, you need to refire the piece with nothing on top of the foil so that the enamel below it can grab onto it and flow into the voids underneath. After that you add another layer of enamel, fire that, then in the instance of cloisonné that’s the layer you’ll set your cloisons onto and fire them into.
I’ve just been learning this myself, so it’s top of mind. Thanks for the opportunity to infodump.
ETA: are you familiar with piercing the foil to help prevent air bubbles under it? I’ve only done small pieces of foil so far and haven’t noticed a problem, but I read that it can happen with larger fields. I read that you can either use a pin to poke a few tiny holes, or lay the foil over a sheet of sandpaper and gently roll over it once with a brayer or dowel. When held up to light you’ll see tiny holes all over, but when laid flat they’re invisible.