Absolutely! Actually, what you're wanting to do is an excellent way to add functions conveniently. All you have to do is use Notepad or some other text editor (Notepad REALLY will do).
1. Copy your acad,mns file to a safe filename in case of disaster...
2. Open acad.mns in notepad.
3. Use Find to locate the menu area dedicated to the pop menus. Find...pop0 (that's popzero). (Also look at pop501. I think you'll be VERY interested!)
4. Look up and down the pop0 menu to see how they use command names and their options to perform particular functions. Add your own function using the same format. Remember, spaces mean "return", etc. Also, that stuff (code)on the left edge is useful, but its not necessary for your particular situation right now. Just type in your command like other commands on the menu, along with your favorite option (seperate with a space: command space option [no need for a space or semi-colon on the end of the line. ACAD inserts the necessary "return" at the end of the command line). Save your work. (you don't need to close it in Notepad. I just save, and keep it open in Notepad for more work, like debugging, etc.)
5. Inside ACAD, use "MENU" to reload the newly modified menu. Choose ACAD.mns (NOT ACAD.MNX...) from the available choices. The menu will reload, will NOT affect your ICONS, and when you hit your middle button your new entry will show up on the pop list. If you're lucky, you won't have to debug your entry. If you DO have to, don't worry. Re-try your function a couple of times and watch what goes wrong, and change how that's typed into your line entry by going back to STEP-2 above.
*** If you completely screw up, simply re-copy your safe acad-mns file back on top of the edited ACAD.MNS file you messed up, reload it like STEP-5 above. Everything'll go back to where it was before you started. You can start over again and go from there.
** READ ABOUT THIS IN YOUR ACAD MANUAL. The one included with the program is adequate for this purpose.**
Hope this helps. You'll prob'ly have more questions like spaces, positioning, auto-escapes, etc.
Good luck!