20 is the administrative distance for eBGP, not its metric. It will be used to choose between eBGP routes and those from another routing protocol. It is rarely a good idea to change administrative distance, but it can be done.
If you used iBGP with the remote sites having the same AS, you could use local-preference.
eBGP won't look at MED or metric by default from a different origin AS. Cisco has a knob to allow this called "always-compare-med" but I don't know the Adtran equivalent if one exists.
For eBGP you can use a route-map and add prepends to de-preference the routes.
You could also set a community and then map that community to adjust the metric on the other end.
If the remote site is comparing the default from eBGP and that from another routing protocol, you'll probably need to use communities.
Be careful redistributing defaults, it's easy to get undesired behavior. A big picture of the problem you're trying to solve with this configuration might help. There may be a better way to accomplish the end goal.