[ho1Aetoo]

I'm not even sure if the dd-wrt GUI supports this at all.
On my Atheros router I can't add a second WAN port via GUI.

But this is all possible via the command line and "swconfig".

And no idea what you want to do, you still haven't explained that to us.

As I said I guess you want to have a second WAN port.
And there is nothing to bridge at all.
The ports are internally connected via switch fabric.

If you have two WAN ports you can use one as uplink to the router and connect a second device to the second port.
The second device then gets a second WAN address.

[toke lahti]

If I still may ask more,
is there any kind of way to automatically document all the settings to some human readable format like text file, so that after installing a new fw, you could use that as a note, what all the changes and additions you need to make for the gateway to function as before?

[egc]

You can save your nvram parameters with:
nvram show > /tmp/nvram.txt

This will make a file nvram.txt in the tmp directory which you can copy

Also a very useful tool is easyddup: https://forum.dd-wrt.com/phpBB2/viewtopic.php?t=322503

But most browser have the ability to print to PDF so what most of us do is just make screenshots that way and use them if we have to setup from scratch.