This is what I use to add items to a selection set
(setq ss ssadd) ;need at least a blank selection set to add things
(setq LastEnt (entlast)) ;set this before you create or modify anything in the drawing.
... do stuff
(while (setq LastEnt (entnext LastEnt)) ;this will add anything created after lastent was set to selection set ss
(ssadd LastEnt SS)
)