I have D-Link dir-600 router with dd-wrt firmware.
I have two more APs belonging to my neighbors.
can I use these APs to boost my routers strength over WAN.
I can have physical access to neighbors router.
If your neighbour has an access point (not a router) that they are not using on their network, and that access point is capable of acting as a wireless range extender, then yes. Said access point will only be a range extender for your network and will not be usable on your neighbour's network.
If the neighbour has a router with built-in wireless, then no, you will not be able to do this.