The Import tool is designed to import posts from the export which do not already exist in the install, ie. as a backup of the blog or whilst being moved to a new installation.
You should really make all the edits from within WordPress for the best compatibility.
Depending on what you're trying to accomplish, you do have a few other options: Delete the posts you've modified, then import the import file, this *should* import the changed posts, as long as the export file is still in a valid export format. Alternativly, if you're doing a search-and-replace on the posts, you may find that a plugin such as Search And Replace would be a better choice.