Same here. I'll keep you updated if i find a way.

Would love to load my current island into the island creator and tweak it :)

Well the best I've been able to do is load in the default version of my island (like how it looked before I touched it) After doing so, it corrupts the file and makes it un-loadable again.
It looks like people have asked about this on Discord and Devs are been real quite on the subject. I believe there is some differences in the Hexcode inside the .json files when in comes to in game islands and customs islands. I have yet to figure it out, but haven't given up yet.

Yeah it's inpossible at the moment. Maybe later.