Announce prefix to BGP conditional

Hi Is it possible to announce additional prefix in BGP (let's say inject it) conditional ? My configuration is 3 BGP sessions: 1st BGP session to ISP 2nd and 3rd BGP sessions to customers I would like to announce default-route (0.0.0.0/0) via BGP only if I receiving let's say defined 2 prefixes from my ISP. Is it possible at BIRD ? How I can do this ? My ISP will NOT announce default route to me, so I cannot simply re-announce it to my neighbors. Rob