Being as the Outlanders rules pg 28 make no special mention of how to handle downed zombies, I would say they do crawl around because I can't think of any reason that you would not treat them like any other ganger.
They can't suffer flesh wounds and you can't pin them, so until they roll a 1 or reach a ganger to attack them in close combat, I would say they just crawl around.
Also since they can't suffer flesh wounds it is safe to assume that they do not suffer the -1 WS/BS penalty when they get back up.
One last thing I would suggest is to remove their 'always charging' rule when they crawl into base contact with a ganger, but that is just opinion, I have nothing in rules as written to back this up. As was pointed out in the comments by Paul Marshall, if a zombie did this, they would be removed during the combat because when a fighter is downed as a wound result in close combat, they are automatically removed.
I looked high and low for an official ruling and could not find one, but from a discussion about it on another forum, it seems that most interpret it this way. There does however seem to be some contention over weather or not they stand back up if they roll a 1 during recovery.