Can you do category extensions (achievements) with the game 100%?

Most CE runs can be done with a completed file. A lot of them are timed differently than when the achivement appears, but the route is the same. Check the rules for the individual runs to see what is required. For example, Chirp requires the achievement to show up, whereas something like BSoD has you go to where you would get the achievement, but is timed differently. It's category dependent.

Thanks for the reply! But I found a way to save my game so I can restart.