Hi Luc Baeten,
The administration of Sitefinity can use only one set of providers at a time, it is not possible to manage permissions for two role providers at the same time. Page permissions are not an exception, even though you need to have only "view" rights for the other set of users. The easiest way would be to add the administration roles in your provider (if this is an easy task in your case).
There is another possible solution, however, it takes some more effort. You can create a custom page security implementation, parallel to the default one.
A possible way is to create a "permissions" table in the database that holds information about the pages that can be accessed by a given role. Then create a control that sets permissions per page for the roles in your role provider. Eventually you can create a custom http module and on the "post authenticate" event to check if the user has permissions for the desired page. To make it compatible with the Sitefinity security, you may set "anonymous access" for all pages to "deny" and then give "view" permissions for "everyone".
This way users from the default Sitefinity database will manage the CMS, based on their (Sitefinity) roles, depending on your implementation they would be able to manage page permissions for other users. The other set of users from your custom providers will have "view" permissions for pages, depending on their roles.
However, I think this won't go smoothly and don't recommend it, unless it is inevitable. Sorry for the tentative answer, if you have any other ideas, we'll be glad to discuss them.
the Telerik team