Can this table have a FK?

In SQL Server 2005, I've added a new table: TableNew.  In an existing table, I've added a new column that is a reference to TableNew:

    OldTable.TableNewId

TableNewId will have to allow null since there are records in the table.  New records will add a value in this column. Since the column allows null, is it possible to create a FK to TableNew.TableNewId?  Is that needed at this point?
brettrAsked:
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Ephraim WangoyaConnect With a Mentor Commented:

You can create the key but you cant have any referential integrity checks
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brettrAuthor Commented:
What good is the key in that case?
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Ephraim WangoyaCommented:
Precisely
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