That is a difficult challenge and can only be done with compromises.
If you really do need the foot orthotics for your problem, then it may be a matter of changing your preference in shoes to one that they can be worn it. You could go back to the ballet flats later.
If the problem is relatively minor you may be able to get away with something like these:
The are a silcone self adhesive pad that is stuck in the footwear if foot orthotics can not be used. They are not as good as foot orthotics, but can be used to trial and as a compromise if there is not enough room in the shoe. I use them a lot in ballet type footwear, cycling shoes, football foots and high heel shoes.
Posted 453 days ago