Can I add a DMM to a 34980A that was purchased without the DMM?

No, a DMM can't be added later to a 34980A that was purchased without a DMM. The labor to disassemble the 34980A, install the DMM, and reassemble the unit together with the low cost of the DMM make this impractical. No trade in credit will be given, so if there is any possibility that the DMM will be desired later, it should be included in the original 34980A order.