There are several ways to do this.
Logical Systems (LSYS)
Logical Systems are a means of segmenting your Junos router into multiple logical systems, which have separate configurations, routing protocols, and interfaces.
This is similar to having separate routers that happen to share a physical chassis. The neighboring routers, such as your BRAS, have no idea there is any overlap among the networks.
To connect the LSYS together look into Logical Tunnel (
VRFs / virtual-routers / L3VPNs are another means. You could configure the smaller network as a VRF or L3VPN.
You can do this without support for L3VPN on your BRAS by treating it as a CE device. It can have ordinary routing protocol sessions for e.g. BGP or OSPF.
To share transit or peering, you can leak routes between the main routing table (called
master by Junos) and the VRF/L3VPN table(s).
In truth, though, if your goal is to consolidate the equipment of the other "ISP" into your own, what you should really do is merge them into a single AS. This will simplify your network management in the future. It's not clear why you don't want to do that, but I see you've commented that you don't.