I believe you can do this using the manual prefix addition with the "no check" option.
For the above command to work, you need to have one active route for this prefix in
the local routing table. However, if you do not have any active route for the prefix
to be advertised, use the following command: FW(untrust-vr)-> set protocol bgp FW(untrust-vr/bgp)-> set network 172.16.1.0/24 no-check
http://kb.juniper.net/InfoCenter/index?page=content&id=KB5940