When you open a new company, only the fiscal numbers of municipalities from Genève and Vaud are proposed.
If you need to add codes from other cantons, you can't use the postal codes but need to use the fiscal codes.
To insert new municipalities, check here.
If you just need some, follow the above procedure.
Below, two downloadable files :
1 - An Excel file allowing you to find a municipality according to its postal code. (several postal codes can be attached to one fiscal number)
2 - A text file containing the whole list of Swiss muncipalities ready to be imported in References - Communes. (Don't do it if you just need some, it's heavy).