Martin:
You can try this: open a new blank file and draw a wall with the one window you want to transfer. Save as... this file as an archive (.pla) file. Open this .pla file, and when the "Open Archive Project" dialog box appears, select the "Extract elements to a folder" option, this will create a folder with the window and its associated macros. You could then add this folder to your standard library.
A number of caveats: I think that ArchiCAD is searching the scripts of the library part for "CALL" statements to find the macros, this "CALL" statement is not required, so ArchiCAD could miss some macros. Also, I do not know if ArchiCAD is then searching those macros for "CALL" statements, so nested macros could be missed. And this from the GDL Reference Guide:
Warning: If string variables or parameters are used as macro names, the called macro may not be included in the archive project, unless the “Include All Parts of Loaded Libraries” option is checked. A greater problem is if these macros have the same name as other macros in your standard library, but do not have the same code, ArchiCAD may end up calling the wrong macro. So this may work, but will need testing.
HTH
David