Welcome to CH. Just wanted to give you a heads up that, not many have hacked GPS's here, or at lease since I started in 2006, its very infrequent that anyone requested info on a GPS, let alone hacking one is a first here that I know of.
GPS's are pretty much throw aways, especially these days, so when its no longer supported you can simply buy a new one for like $50 or get one already integrated into a modern phone. You would have better luck finding or making an app for a phone that has a GPS in it that can be interfaced.
Unfortunately the easiest way to hack this device would require using illegal tools that I can not state here and do not advise using at all. It would require decompiling the software that interfaces with the device to see how it communicates with the device and would require you to be proficient in the language that it was programmed in. Then recompile an alteration of this interface software which is extremely illegal.
The only legal way to interface with this device would be if someone wrote from scratch without decompilation of the original software, used in the process to make and open source tool to use that is not in violation of EULA and generally non competing. For this to happen you would pretty much need someone who was part of the original team of programmers on this project available to assist, because otherwise you can program all you want, but its like trying to pair up a key to a lock and the lock is unknown type and tooth combination.
If this was a device that boots with ability to serial term it without authentication required then alteration would probably require a reflash of it, but to do that you would need an altered custom ROM for it, but the likelyhood of such a ROM existing for it is very slim.
Best of luck with this, however I dont believe anyone here will have a solution for this device. Didnt want your post to sit at 0 replies thinking this is a dead forum, its very active, but your request is outside of the norm.
My suggestion would be to use a modern phone that has a GPS and of which apps can be programmed for it to interface with this more easily than that of a regular off the shelf GPS. Best of Luck and hope to see you back again sometime
