Pigtail off before kitchen 20A GFCI for Cabinet Lighting?
No.  A circuit which supplies kitchen countertop receptacles can ONLY serve: 


Receptacles located in the kitchen or dining area
A clock on the wall (remember those)
Auxiliary electrical loads in a gas powered oven/range


Even if your installation is grandfathered, you're not allowed to make a grandfathered installation worse than it already is. 

However, kitchen receptacle circuits can certainly serve receptacles in the kitchen under or near a cabinet.  Are you sure that low-voltage LED lighting doesn't come with some sort of power supply that plugs in?