Hi!

I've played around in the commissioner settings but I can't seem to find out how to export the rosters of my league and successfully import them when making a new league. I know how to import them, as you are given that option when you create a new game. However, if anyone knows about how to export them, please let me know!

If I understand correctly you have an existing association your exporting from. Then are you trying to import back into the same association? If so you do that in the commissioner just below export. You should be able to import your change back in to the same association.
The file you can Import at the creation of an association is a different data file containing more data than an export, if that's what you're trying to do.