Hi,
You could import interface-routes in inet.0 using rib-groups:
set routing-instances isp1 routing-options interface-routes rib-group inet isp
assuming next-table is being used in the RIs to direct return traffic to inet.0
You could repeat the rib-group config for the other RIs.
This one is not really required as the interface routes from inet.0 are not required in the other RIs:
delete routing-options interface-routes rib-group inet isp
Cheers,
Ashvin