FOSS gamedev and creative worlds

Can I apply a mod to an existing world?

Mods apply to the whole server.

Sure.

Adding a mod usually works fine. You can even remove it later on and resume playing after a server restart. The only drawback is that objects defined by the mod you removed will show up as "unknown object"/entity ingame (like the "hacked" air one sometimes gets).

Some mods add their content only to areas that are loaded for the first time; thus your new objects may never appear as long as you don't visit areas you havn't been to before.