This page on server hardening has a large section on adding a SWAP partition.

Why is adding a SWAP partition good for server hardening?

Yes, it is possible.  However, that runs the risk of destroying the device without getting the data off first, which is undesirable.  It also does not achieve the political goals of forcing Apple to assist in decrypting the device, paving the way with precedent for the flurry of future requests of this sort to come, some of which are certain to have less favorable facts and thus are not as suitable as test cases.   