; with
UserID as
(
	select	distinct UserID
	from	tbl
),
WeekID as
(
	select	distinct WeekID
	from	tbl
),
ItemID as
(
	select	distinct ItemID
	from	tbl
),
comb as
(
	select	UserID, WeekID, ItemID
	from	UserID
		cross join WeekID
		cross join ItemID
)
insert into tbl (UserID, WeekID, ItemID, ItemQty, ItemYTDQty)
select	c.UserID, c.WeekID, c.ItemID, ItemQty = 0, isnull(y.ItemYTDQty, 0)
from	comb c
	cross apply
	(
		select	ItemYTDQty = sum(ItemQty)
		from	tbl x
		where	x.UserID	= c.UserID
		and	x.ItemID	= c.ItemID
		and	x.WeekID	<= c.WeekID
	) y
where	not exists
	(
		select	*
		from	tbl x
		where	x.UserID	= c.UserID
		and	x.WeekID	= c.WeekID
		and	x.ItemID	= c.ItemID
	)