This is a good question! I haven’t tried to do that myself, and a quick google search didn’t really turn up anything. So I’m not sure. D: I use quite a few custom collections myself, but I use the “organize collections” option in-game to move all my custom ones to the front of the catalogue and rearrange them in order of how frequently I use them, etc. (OMSP collection is always number one!)
The organize-collection feature is kind of tedious (you move the collection up or down the line one click at a time), but if you have all your custom ones in the beginning, you won’t have to pass all the Maxis ones to find something. 🙂