Yes, unfortunately there is a limit of ten on the file-open-other users folder list you are talking about.
You could instead add all the users to your folder list, but first they have to add you to permissions on their mailbox, the top level. Reviewer permissions are all that are needed. Then you can add them to your folder list. Go to tools – email accounts, leave it on view or change, click more settings, click the advanced tab and add them there. That is for Outlook 2003, and you did not mention what version of Outlook you are using.