Finally figured it out. It can be done. In order to add a new mission to DISH you need to follow these steps:1./ Create the new txt, mis and dta files and name them appropriately. For example, usah11.txt, usah11.mis and usah11.dta. Place these files in the missdata folder.
2./ In the missdata folder, delete (or rename if you're paranoid) the file called missions.blk
3./ Start up DISH. Presto! Your new mission is now available.
I've added the above example to my DISH setup and it works perfectly. I took the stock airfield take-off training mission and edited it to take place at 23:00 hours in heavy rain.
It seems as though if you delete the missions.blk file, DISH parses through all the mission files available and rebuilds the blk file and hence the mission selection list. If you like to try my example you can download the txt, mis and dta file here:
http://members.xoom.com/PsychoJon/dish/usah11.zip
Simply copy my new files into the missdata folder, delete the missions.blk file and start DISH. I think this could open up worlds of re-playability potential for this sim. Now everybody start studying those DTA files so that we can put together a full featured mission builder.
Jon