Hi,
The answer is yes. If the provider hand-off is ethernet you can use any of the available ethernet ports. However, I would recommend you create a new zone (i.e. WAN) and bind the interface to it. This will allow you to use policies to control traffic. Also, by default your trust-vr will be used. This can be changed, but it's important to note this (especially if you have a default route via another carrier).
-John