Client-side includes is the terminology, but it's not as straight-forward as just including a file. (The link discusses the process, and advises of some pitfalls.)
Easier would be to use an **iframe **to include content at a specific point in your document.
If you decide to pursue a copy/paste approach you could consider adding specific comments at certain points in the files and use ST's Find/Replace in Files feature (or some other file utility) to replace (or append after) the comments in one fell swoop. I think you would need to plan this carefully, and I would consider something like this:
so that you could insert the text between these comments and repeat the process at a later-stage if you needed to. Andy.