How do I force my Graphite Airport to distribute 192.168.x addresses instead of 10.x.x addresses?

How do I force my Graphite Airport to distribute 192.168.x addresses instead of 10.x.x addresses?

I'm sharing a single IP address but there's no way to change what network it uses for distributing the private addresses via DHCP. My problem is that my VPN to work already uses the 10.x.x network, so I'd like my home to use 192.168.x.