Despite the earlier negative comment, I can't see what is wrong with this, but I also can't see what Jeppesen has to do with it?
Jeppesen publish a set of standard navigational data, as do various other providers. All the nav boxes that I have used include the ability for the operator to add their own additional data to the set that came from the nav supplier. You don't change the underlying base data in the process, and you certainly don't violate any laws.
There are hundreds of "company waypoints" added to the FMS in the jet that I fly, none of which was deemed important enough by the database vendor to include, but which we routinely use as real waypoints. There are even a handful of complete instrument approach procedures, meticulously transcribed from the AIP because again, the database guys keep missing them out.
More unusually, we can and do add entire airports (it's to do with database restrictions, but I won't bore you).
In light aircraft fly-ins to farm strips and private airfields it is quite routine for the destination to be missing from the standard aviation database - no problem, just add it yourself. Of course, a prudent flying club will then compare GPS readouts on the ground before leaving the clubhouse, to make sure all the pilots are going to the same field!
I am quite certain (although I am not a user) that the Garmin 296 will allow you to do this.