It will be really hard because map scalars are everywhere, and if you plug 3 bar map ad voltage at 100kpa will be 1/3 of 5v I guess which will be 1.67 volts and from 0 to 1.67 you get very low resolution, unless you drive boost or near boost all the time, reolution will suffer.
Much easier will be to add second 2 or 3 bar map sensor on one of the free ad channels, make new 3d table with bpw adder or whatever you like. That way you won`t lose resolution off boost and can get full control on extra fuel needed. Also you can map spark retarder that use the second map sensor.
Of course we will need full graph with AD counts vs boost on the new map sensor so it can be dialed in proper way and a volunteer with engine to sacrifice, Initial test may be done with variable potentiometer hooked on the ad channel on NA engine