What you're looking for is something called client mode or wireless bridge mode (these are two different ways to do this), and it does seem to be supported as wireless bridge mode on this router - this needs you to set 2 APs as wireless bridges, and it seems you can't use either router to serve wireless clients when you do this - see page 86 of the manual
Alternate firmware might be an option, but outside a WIP openwrt build, I can't find any options - ddwrt's client mode is what I'd use for this but its not hugely common
It seems repeaters would do this - acting as both a bridge and a transmitter and offering up a ethernet port for whatever you need. They're also pretty cheap and cheerful.