You can add additional resellers by following these steps.
1: Log into Apple Business/School Manager
2: In the left column, select your name at the bottom and choose Preferences from the pop up menu
3: In the middle column, select "MDM Server Assignment" in the Your MDM Servers section (do not select an actual MDM server)
4: In the right pane, at the top, will be a section titled "Customer Numbers." Press the Edit button associated with that section.
5: From the popup menu, choose Reseller Number, enter the number in the text box, and press the Add button.
6: Repeat for each additional reseller.
Please note, this will not automatically associate prior purchases. Reach out to the reseller and provide them with your Organization ID. Then ask that they backload prior qualifying purchases. Remember, retail purchases cannot be associated through auto device assignment. On T2 or higher you can use Apple Configurator to associate to ABM/ASM. However, a wipe is required.
Hope this is helpful