T-SQL: CTE with identity columns

I'm building a tree (bill of materials style), and transforming some data. Consider the following table:

BillOfMaterials

• BomId


• ParentId

Now I'm using a CTE to fill it up:

with BOM as 
 (
 select @@identity as BomId, null as ParentId <some other fields> from MyTable
 union all
 select @@identity as BomId, 
        parent.BomId as ParentId,
        some other fields
 from MyTable2
 inner join BOM parent on blabla)
 
 insert into MyTable3
 select * from BOM
 

Problem is: the @@identity will only give me the identity of the last record inserted before the union.

What can I do to get the identity? I can modify Table3 but not Table1 or Table2

I know I can use a GUID, is that the only option?

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