Deformation Sets editor...
Find out which set belongs to the lattice you want to add more object/points to (see what hangs off latt. ffd node in hypergraph) ...then just add point to it (prolly easiest to just select whole objects, or a subset of their points in the viewport, select set, and hit the + button in the editor. Remember to ware what your latt.Base shapes are up to...might want to select their nodes so they show in the port.
G.luck,
C.