If you know the coordinates, you should be able to punch them in manually. Your unit should recognize them, but if it doesn't ... you can put those coordinate numbers on here : GPS Coordinate Converter, Maps and Info and it will convert them to other formats, one of which will work.
I had to do that on my Lowrance Elite 5DSI unit. The GPS coords that our F&W website has, for the cover they've dropped in our lakes, were not recognized by my unit. I put them in the converter and then used the "GPS" format numbers to manually punch in the waypoints. Went to the lake and found the cover right where the waypoint icon said they'd be !!
... cp