OK, I got it. Here is what to do:
Right-click the toolbar below the menu bat and bring up the Edit GDL Library Parts toolbar.
There use the Open Object by Subtype command.
In the appearing Subtype Hierarchy Dialog, select the Frame subtype you want to modify and Open it for editing.
Save a copy of it under a new name.
Edit it to you liking.
As you can see in the attached image, there are several Frame subtypes so you will need to check each to see which one you should start working with.
Get Archicad Tips at https://twitter.com/laszlonagy
AMD Ryzen 1700X CPU, 48 GB RAM, Nvidia GTX 1060 6GB, 500 GB NVMe SSD
2x28" (2560x1440), WIN10 PRO ENG, AC20-AC26
Loving Archicad since 1995