You don't have to. GooTool does it for you. Just make it a .goomod, and the button will be there in Chapter 1.

I have a feeling, however, that this isn't what you're asking. If you want to add a button to play a level inside another level or in MapWorldView, it's not possible. However, if you want to add a level button in another chapter... be patient. davidc is going to add features to do this to GooTool later in the future.

Or you can do it with merges. The moderators have said that's okay.