Hey Michael

Thanks for posting in the Community! Unfortunately creating sub-libraries within your custom library isn't yet possible in Lucidchart. To break up the library you would need to make new libraries and move the shapes over. 

That being said we're always looking for ways to improve Lucidchart and your feedback is the best way for us to do that. Would you mind adding your idea to our feature request form? Ideas submitted to this form are passed on to the product development team for their consideration.

Looking forward to seeing your idea in Lucidchart--let me know if you have any other questions! Cheers!

Sure