Step 1. This first requires that you edit the cmdicons.grp and add an entree at the end of it, recompile it and put it in a cwad.
Step 2. In the Stargraft\data directory there is a file called 'icons.dat', open it.
Step 3. Add a couple extra lines to the end of the Dat file and save it. (note: you should probally should save it as something different if you only want to use it for just that custom)
Step 4. Open Stargraft and find the button you want.
Step 5. Right click on the icon part and click 'Load String'.
Step 6. Find the icons.dat and click 'Open'.
Step 7. When done save and run.