Certainly not manually, but automatically. This does require
some type of "if/then" logic in your router (ie, if you don't see
this route from this source, then announce these routes to xxx).
If your router can't do this, then you need to talk to your
router vendor*. One needs to define quickly, but the time it takes
to propogate a route isn't that long.
* - tell them they need to do things like this and build a router that
will handle several 100k routes.