If memory serves me correctly, the m148 uses several sets of 3x6volt batteries wired in series (creating 18 volts), so you'd need to come up with a power source that would supply it with the correct voltage(s) -- the m148 charger (again if memory serves correctly) supplies 48v of power to the unit for charging.
I have a feeling that the hardest part of successfully converting a m148 to external power will be coming up with a reliable charging system for the external batteries. This has left me scratching my head and periodically replacing the m148's internals. I can get full day runtimes with my mk4>kc5>pfa setup on fresh-ish m148 batteries. If yours are no longer keeping a charge, battery replacement is a fairly straightforward process