Do you like create object in block definition or copy an object to block definition?
If you have block name:
; Get a pointer to block definition by block name
(setq blk-def (vla-item (vla-get-blocks (vla-get-activedocument (vlax-get-acad-object)))))
; Add a new AcDbLine object to block definition
(vla-addline blk-def (vlax-3d-point '(0. 0. 0.)) (vlax-3d-point '(10. 10. 10.)))
If you want to copy object, use vla-CopyObjects method