Not too hard, try this:

Find-what zone: Word\x20$
Replace-with zone: Word,
Search-mode: Regular-expression

You can use a plain old space instead of the \x20 but I like to use \x20 here on this site because it is more visible. Either works when you are actually doing it.

So in the spirit of learning something, the $ is the key element here. It basically means end-of-line…so you are searching for W…o…r…d…space…followed by nothing else on the line. Replacement affects only those cases where a search match occurred, obviously.