You can't do this with Crystal but you can buy or develop your own viewer application (using the runtime components that are included in Crystal Developer).
For a list of 3rd-party Crystal viewers see:
My DataLink Viewer allows you to launch another Crystal report by double-clicking any section in a report you are viewing. A formula you place in the double-clicked section provides the information about the report to launch, the parameters to pass to the launched report, as well as the User ID & Password (if not shared with the initial report).
By the way, if you maintain a listing of reports in a database table, you can create a report that acts as a "menu" for launching these reports. In other words, this type of functionality has other benefits besides overcoming the subreport-within-subreport limitation.
Cheers,
- Ido
CUT, Visual CUT, and DataLink Viewer:
view, e-mail, export, burst, distribute, and schedule Crystal Reports.