Is there a way to make ham in a casserole less salty?
You need to add bulky things with no salt, so that it'll average out to a saltiness you're okay with.

Although cream of mushroom soup sounds like a nice easy way to add some goopiness, it's probably pretty salty. If you can find low- or no-sodium soup, that'd be a much better choice. That kind of thing might be easier to find in fancy/organic/"healthy" brands. If you're willing to make your own soup/sauce, you can easily make it salt-free, but it sounds like you're looking for quick and easy options.

You could also try adding more vegetables and noodles without salt. (Starchy things like potatoes can also work well, but maybe you prefer noodles since they're already in there.) If you end up needing more goopy stuff to hold it together, again, try to find something sodium-free or low-sodium.

But there's not really any avoiding the main issue: you have to add a bunch of stuff without salt, so you're kind of going to have to turn your casserole into a bigger casserole.