Can a C6250 be configured to receive input from Ethernet instead of coax?

> Now becuase of coverage and lack of Ethernet ports [...]

   Poor _wireless_ "coverage"?  From what I can see, a C6250 won't give
you much in the way of Ethernet ports (unless you add an external
Ethernet switch).

> My question is Can I configure one of the Ethernet lan ports (or usb
> for that matter) to be an input for the c6250?

   Forget USB for networking.  As a router, I don't see a way to make a
LAN port into a WAN port on this device.

   Is this double NAT possible and does it cause problems?

   Stacking two routers can cause NAT problems, but you're safe on that
front if you can't configure an Ethernet WAN port.

   I don't see an official setting for wireless access-point mode, but
you should be able to do it the old way, without the special setting.
Assign some suitable LAN address to the C6250 LAN interface, so that
you'll be able to talk to it later: ADVANCED > Setup > LAN Setup : LAN
TCP/IP Address : IP Address.  (Here, "suitable" means something
available on the real router's subnet.)  Disable its DHCP server:
ADVANCED > Setup > LAN Setup : Use Gateway as DHCP Server.  Connect one
(either) of its LAN ports to a LAN port on the real router.

   In that mode, the wireless radios and the remaining Ethernet port
should be part of the real-router subnet.