It's definitely not, and in fact swap on a modern OS doesn't work that way. Something can be only in physical RAM, only in swap, or (when "clean"/unmodified vs the copy in swap) in both (allowing it to be dropped from physical RAM quickly). So one obvious approach would be to simply use the external RAM as swap.
Relatedly, I've heard of using Intel Optane as "slow RAM" for cold pages, and I think the idea there is also that it'd be in one or the other but not both. (Optane can be thought of as very expensive/fast flash or very slow/cheap RAM.)