Alas, it's not so easy - I was able to add online multiplayer to this selection of games because they are all freeware titles, and the developers had published the source files for them somewhere.
While (with a lot of time and effort) it is possible to mod some games even without having the associated source code, it is highly impractical, and I think in such cases it should be up to the developer of the game to hire someone to do online multiplayer implementation (moreover if the game is commercial and stuff).
Additionally, the original Binding of Isaac is said to be made with very outdated tools, noticeably lowering the odds of it being possible to do anything new with it, even if the developers were to decide to do something with it. No idea about the new one.