When you create the OPX, it acts like any other single line extension. You can dial another extension, or press 8 (or whatever the feature code is) to access an outline line. If there is no need to dial extensions on the 5000, they can be set up as house phones.

Since you’re not using a PRI to connect the systems, the configuration will be for a plain T1 - there is no D channel. Simply use a T1 cross-over cable to connect the two systems. I actually preferred to use regular patch cords and do the cross-over inside a dual biscuit jack.