Configure I2P declaratively

I don’t understand how to set up I2P in a declarative way.

The only two possible options are
environment.systemPackages = pkgs.i2p
or
services.i2p.enable = true
but I found no way to declare the values that should be set in the configuration files
/etc/default/i2p and /var/lib/i2p/i2p-config/router.config

There are a lot of options for i2pd, but none for i2p