Transact SQL :: Get All Children Of A Product With Product ID In Result Set

I am using the code below to get all the children of a particular product and it is working fine. How to get the particular product's id in the select statement. for example, i need to show 891 in a separate column for all the records returned by the query below.

DECLARE @Hierarchy TABLE (Product_Id INT, Parent_Product_Id INT)
INSERT INTO @Hierarchy VALUES (123, 234)
INSERT INTO @Hierarchy VALUES (234, 456)
INSERT INTO @Hierarchy VALUES (456, 678)
INSERT INTO @Hierarchy VALUES (678, 891)
INSERT INTO @Hierarchy VALUES (891, NULL)

[Code] .....