If my pumpkin pie recipe calls for 3 eggs + 2 yolks, can I use 4 whole eggs instead?

They are also good for the dog's coat... As HLA indicates, sometimes there are just so many it is overwhelming (of course, that can also be an excuse for Angel food cake, or any of an assortment of meringues...)

Extra egg whites can be frozen in an ice-cube tray, then popped out into a freezer bag, saved for those days when you just have to make a butterscotch meringue pie...

It will also change the texture and diminish the richness of the custard.

Yes. The more white in the custard, the more "eggy" it will taste. There's also no law that says you need to save the extra whites - sometimes when I'm doing large scale baking I have so many extra whites I end up flushing them down the toilet. As yet I've not died from doing so.