How Can I Assign Data Returned From A Stored Procedure Into The Return Table Of A Table Valued Function

Here is the scenario,
I have 2 stored procedures, SP1 and SP2

SP1 has the following code:

declare @tmp as varchar(300)
set @tmp = 'SELECT * FROM
OPENROWSET ( ''SQLOLEDB'', ''SERVER=.;Trusted_Connection=yes'',
''SET FMTONLY OFF EXEC ' + db_name() + '..StoredProcedure'' )'

EXEC (@tmp)

SP2 has the following code:

SELECT *
FROM SP1 (which won't work because SP1 is a stored procedure. A view, a table valued function, or a temporary table must be used for this)

Views - can't use a view because they don't allow dynamic sql and the db_name() in the OPENROWSET function must be used.
Temp Tables - can't use these because it would cause a large hit on system performance due to the frequency SP2 and others like it will be used.
Functions - My last resort is to use a table valued function as shown:

FUNCTION MyFunction
( )
RETURNS @retTable
(
@Field1 int,
@Field2 varchar(50)
)
AS
BEGIN
-- the problem here is that I need to call SP1 and assign it's resulting data into the
-- @retTable variable

-- this statement is incorrect, but it's meaning is my goal
INSERT @retTableSELECT *FROM SP1

RETURN
END