Making your own levels is not difficult. The file format can be deduced pretty easily from the levels supplied, except for complicated things such as covers, backgrounds and guns, which take a little more thought.
This is an example of how to make a level in c.215.6 seconds. It shows the basics. The file can be found in !Tremor2.Data.Levels.My_Level, and you can muck around with it as you wish. The file !Tremor2.Data.Levels.!!Guide is the reference guide to what the letters do. The password, so that you can test your marvellous creations, is mine. Yes, I did mean M-I-N-E, not that it belonged to... oh, never mind.
This is not an explanation; it is simply what the numbers are. Deduce how it all works by trial and error, if you're so committed.