I own a Lowrance so I'm not really up on the Navman, but since you haven't had ANY replies, I know what it is like waiting for help. For what its worth, here are a few of my thoughts.......... remember, I could be totally off the planet here.
Since Navman only use the C-Map cards, I think that is your only option. There is another card, a C-Map USER card (as opposed to the map card), which is for storing user data.... you could use one of those. If you could get another card I would be trying that first, rather than maybe stuff up your map card.
Another option worth investigating is to look at some of the free GPS software, which may be able to interface directly with your unit. A good one is GPS Trackmaker, (www.gpstm.com) which you could check.
No guarantees!
Hope you get a solution.
Mike