Well, to get a permalink id you are going to have to integrate the posts into the WP system, meaning you'd then have to edit your theme to eliminate those posts from the front page and also hook some custom functions to some backend stuff to eliminate your posts from the admin panels and hook still more functions into the rss system to eliminate them from that.
You could include WP into your other files and use its functions to get consistency in appearance and function, but you wouldn't have permalink id nunbers.
You could create a template page that includes your various pages based on the URL, but again, no permalink ids.
Caveat emptor: You can insert posts (via a custom script) with post types that WP doesn't recognize. WP will ignore these posts, as if they weren't there. You can write your own query to get them out or use the posts_* hooks to modify a query. I've done this, but never in production. I was playing with the system on local server. I don't know what might break if you do this. In other words, I don't know if this is a good idea or not.
Whatever you do, I think you are in for a lot of coding. Probably the template option would be quickest and easiest.