I seem to have found a way to do this:

The only caveat seems to be that all of the Vendors on the Item Receipts need to be the same.

1. Click on Enter Bills from the Home Screen -> Select the Vendor. 

2. This should open a dialogue box about Open Purchase Orders -> Select Yes

3. Select all POs that need to be on the Bill -> Ok

For me this combined all of the POs selected and created one Bill.

For reference, I use QB Enterprise Desktop.

Hope this helps!