Hi Nicolas

Thanks for reaching out on Lucidchart Community!

Unfortunately what you are trying to do is not currently supported in Lucidchart. However 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.

For anyone else with questions about custom shape libraries check out this article: Lucidchart Basics: Access Organize and Share Shapes