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Of course: yes, but must keep the battery powering, because the multimeters input common not connected to battery negative. If you can make some reverse engineering to discover the type of A/D converter and find a datasheet, most of case possible connecting the measure input common with supply negative if You apply an additional negative supply.
transformers are great. they isolate electricity, and they change voltages up/down.
as long as you are using a transformer then convert that to DV,a nd regulate it down (9 volts) then the meter will be fine. the transformer isolates the ground from power ground, so the multimeter can do its job right. if you were to just put a small 12v transformer, cap and bridge ud be ok, with a 9v regulator. you could also just use a 9v DC power adapter and put it inside ur box. its the same idea, since its all runninf from 120v ac, and ur using transformers to isolate from the other voltage anyways.