At present it is not possible to create a modal tooltip. A couple of people already expressed the same request, so it was logged in our TODO list for the tooltip. It is very likely that modal functionality will be added to the tooltip soon.
To achieve the same effect now, you can use the tooltip beforeshow and hide events to make a page modal while the tooltip is visible on the page (of course, that would require adding a bit of code too).
the Telerik team