Here is something I hadn’t thought about. Adding different clay (even the same type) to old/completely dry clay will cause cracks.
I used my own formula of air dry clay and added it to a piece on the inside to help strengthen the structure. However I came back to take a look 12 hours later, and the entire outer (dry) shell had cracks all through it.
(Insert photos here)
The two formula’s that I used were basically, but not completely the same. This means that there are enough differences that you will be making problems for yourself. The alternative is that the clay when completely dry has very little give in it.
Either way, it seems risky.