Outlook OWA: Attach files files from different document library Sharepoint Online

Hello,

For a customer of ours we have the following challenge:

If the customer tries to attach an attachment from Outlook OWA 365 from a cloud location, only the default 'Documents' library is available. The customer also has 5 other document library's which are not visible. How can we make the other document library's visible within Outlook OWA so he can get t the files he wants to attach?