Can I use htaccess to insert code into all pages into a directory? (When viewing, not physically inserting code into the document)

If so can someone give me the code for that?

You have to use SSI for doing that but i might have misunderstood your question.

For example, if I upload a html document to a directory, and then view it, it will display the included file. SSI won't work because it would require SSI code on every page.

I was thinking that my upload script can place the code on the page during the upload process, but online editing would be a pain because when the user tries to edit the file, I would have to remove the included file, and then reprint the data after the user submits it's modifications.

What you mean either.

When you refer to uploading do you mean like free hosts? Where when a page is uploaded a banner is automatically placed top/bottom or both on every page?

If this is the case I would be interested in how that is done as well.