Sure, you can either modify it, or completly make a new one with Dragon.
If you want to modify it, the best way is in these steps.
1. copy the Map2.mul to a Map0.mul (and statics/staidx2.mul)
2. edit the map, statics.
3. re-copy the edited Map0.mul back to the Map2.mul
you can find the
how to steps
here
If you are simply wanting to create a new Illishanar map (Map2.mul) with a new one you are going to make, then just draw your map out the same you would on a normal *.bmp, keeping the Map size in a 2304x1600 'box' so you can copy it into the Map2.mul when you are done.
Doing it this way will allow you to run Dragon useing the standard .bmp file size that will make it on a Map0.mul
It is a lot of work, and many different steps to take, but it can be well worth it.