See the following:
http://www.archicadwiki.com/TechNotes/Creating_custom_windows?highlight=%28window%29%7C%28custom%29
and follow the bold note at the top of the page, in other words, all you need is steps 1 through 5 there. Very fast / easy.
When you draw your mullions for the custom window flat on the ground, you can use either the wall tool or the beam tool to create them, whichever is easier. Magic wand the space surrounded by your mullions with a thin slab to represent the glass.
Assign all materials to the elements before saving as an object.
Don't delete your modeling elements. Either do this in its own file, or simply select them all and 'save selection as module' to keep a backup copy of them outside of the project file, or place them all on a hidden layer reserved for such things. Then, if you need to modify your window, you just modify the original bits and save again.
HTH,
Karl
PS See also:
http://tr.graphisoftus.com/downloads/DL's%20for%20GraphisoftUS/Introduction%20to%20Object%20Making/I...
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