You can use Mic-1 MMV to write your own microprograms for the Mic1 architecture. Read the MAL Language Specification for an explanation of the structure of a microprogram.
You may want to create a copy of mic1ijvm.mal and modify it to suit your needs.
If you modify your MAL program, you can reassemble and reload the current microprogram by using Assemble/Load | Current MAL Assemble/Load.
Note that if you add new instructions to IJVM you may also need to modify the instruction set of the JAS assembler. This is described in the section Modifying the JAS Assembler.