Merging multiple routes into a single multipath route

Hello. I am running a setup with 2 uplinks which both have dynamic addresses (via DHCP) and NAT and a Linux router. So I get 2 default routes via DHCP. I would like to use both uplinks via the linux multipath routing and I am searching for a way to "merge" both default routes that I get into one route with multiple next hops. Is there a way to accomplish that with bird? (or any other tool?) All of this is only relevant for IPv4. Greetings Jo