There are likely additional components to connect, and you need to ensure you're using the correct RJ45 jack/magnetics.
You can reference the schematic to see what components are used between the Rabbit 6000 and the Ethernet jack: http://ftp1.digi.com/support/documentation/3001042102_a.pdf
It's probably best to purchase an RCM6710 than attempt modification of an RCM6700 to include the jack. Doing so could damage the RCM6700, and will certainly void the warranty on the product.
I'm not sure how the RCM6710 behaves in the development board and whether either Ethernet jack will work for connectivity. My guess is as long as you only use one at a time it will be OK.