Cynthia - Yes, that risk of having pages updated outside RW by trying to change content from another application like TW is exactly why you shouldn’t do so. RW is, by and large, intended to do all the management of content itself.
I wonder if your last sentence is cause for alarm: how have you already published content from another source?
There’s nothing at all to stop you pushing/(s)ftp’ing pages up to the same server, indeed into the same docroot, as that which RW is managing. You could, for example, take a page Exported by RW and independently and manually change its HTML and content and push it to the docroot alongside the pages published by RW. If it were named differently, RW couldn’t overwrite anything on the server. But it wouldn’t be able to include such pages in its own menu structure (easily and reliably).
But may I (if I understand what you’re planning) suggest rethinking how you want RW to do the work for you? RW really is designed to do all the management of documents and publishing: trying to add and control other docs, while possible, could well be asking for trouble in the long run.