Bill.com provides the option to associate a document with an existing bill.

Associate a document with an existing bill

Once a document has been sent to the Inbox:

1. Click the Inbox tab
2. Locate the document
3. Click the three dots button under the document preview
4. Click Bill under Add To Existing 
5. Select the Vendor
6. Click Search
7. Select the appropriate bill the document applies to
8. Click Save

Note

An eBill cannot be associated with an existing bill.