On this page, you can import a list of already installed maps from the "Beat Saber\Beat Saber_Data\CustomLevels" folder.

To create this list, you can run this command in cmd:
dir /b "C:\Program Files (x86)\Steam\steamapps\common\Beat Saber\Beat Saber_Data\CustomLevels\" >maplist.txt

The result will be in the file maplist.txt