BGP Default route

I'm having a problem with sending a default route. I have two providers I provide a full routing table to through BGP. I'm trying to set up a 3rd BGP session on another interface to a downstream client, so I can take his announced prefixes and announce them to my upstream ISPs. The main issue is they only want me to export the default route (0.0.0.0/0) to them, not the full table. Does anyone have a simple example of this, exporting just the default route to a downstream client, but accepting his prefix (X.X.X.X/24).