CALL OUT OF SUB-ASSY IN LARGER ASSY

For example
the following "Ball Plunger Assy"

CarrLane-22BP1.sldasm

Is composed of the following parts:

CarrLane-22BP1-DEFAULT_22-bp-1-spring.sldprt
CarrLane-22BP1-DEFAULT_22-bp-ball.sldprt
CarrLane-22BP1-DEFAULT_22-bp-body.sldprt
CarrLane-22BP1-DEFAULT_22-bp-lock.sldprt

The "Ball Plunger Assy" is used in a larger assy and is shown thusly in the assy dwg:
See   (It is shown here as an edited text balloon)

CALL OUT OF BALL PLUNGER SUB-ASSY IN LARGER ASSY .JPG

I have in the assy of the "Ball Plunger Assy" given it a custom property of Detail_No. (TEXT) 346 thusly:
See
CALL OUT OF BALL PLUNGER SUB-ASSY IN LARGER ASSY Detail_No.JPG

Now I would like to drop a balloon in the main assy, that has this sub assy of the "ball plunger assy" in it,
That would reference this "custom property of Detail_No. of 346" as I would for a part in this main assy.

Perhaps the only way is to save this  "Ball Plunger Assy" as a PART to do this???

There is no way to do this otherwise???

Note: My customer requires a seperate .xls for the BOM so item / Detail Nos come from there.
but once I plug them in to the custom propertys of the assys be it a purchased as example here or manfu sub assys I
would like SW to keep track and drop the numbers in as I explaned with this simple example.

Thanks