is it possible if i download one of the old missions and then turn them in the a res file in the ME?
Yes, it's possible. However, it's complex. There are a lot of issues to contend with. Unless the original mission is very, very simple, you'll need to do more than just edit and save it. For example, old missions were driven by old versions of Aces High that used slightly different airfield arrangements, so when new versions try to use an old mission, the aircraft may bump into buildings and damage themselves trying to find the runway, and the timing of takeoffs can vary by 1 or 2 minutes compared to the original mission. This, in turn, causes combat encounters to happen at different times and different locations. Often, encounters just "miss" one another, so that the bot planes don't actually engage, but just fly on past. Fixing all of this requires a lot of "tweaking" on any sophisticated mission. You can find a pretty good discussion of the subject (oriented toward beginners with the Mission Editor) here:http://bbs.hitechcreations.com/smf/index.php/topic,295719.0.html
You may also want to know how to fly a different plane within a mission. Look here:http://askmisterwizard.com/phpBB/viewtopic.php?f=11&t=4099