Heya,
The ground unit IDs for skills are within the client itself, they are not found in lub files and you cannot add new ones. These IDs are mostly meant to display a visual effect on the client though. Therefore... if you want a custom ground skill to show an effect, you'll have to use a pre-existing ID from those currently defined in rAthena and work your way around that. Otherwise, if you want to display a custom effect, you'll have to use "dirty tricks". Either way, what you're looking for simply doesn't exist.
But I'll say, what you've described so far is unclear. Most custom skills do not require an unit ID to begin with.