Can I edit a Java mod with Mcreator?

1) Making a mod with Javascript is impossible. I assume you meant Java.
2) No, you can't continue it with MCreator. You will have to remake everything (except the models and the textures) from the beginning. But I advise you to keep modding without MCreator, as MCreator allows less things that handed code.

I do not think this is possible. Best thing I can think of is taking the files of an MCreator mod and putting it with the other mod's files. Though, Im not too familiar with this sort of thing when it comes to Minecraft, so I dont know if that would work.