Even bridge is incorrect term for what you seem to be looking for since wifi bridge is generally understood as connecting a wired network to wifi. I don't think that doing wifi-to-wifi bridging with a single radio is even possible.

In any case N900 cannot operate in basestation mode (read through the "N900 as wifi hotspot" -threads for more info)

Then there is mesh networking where all the nodes route traffic for each other and there can be multiple paths to the internet from the local mesh.

Ad-Hoc mode mesh networking could probably made to work on the N900 (in fact I intend to look into it in the Mobile HotSpot project when I happen to have time and inspiration) but it will require mesh protocol support from your wifi basestation/router as well.