"I generally add a DC voltage to measure DC currents in the opposite direction. Values above the DC offset is positive, and below that is negative. "

When DC currents in both directions are to measured, (like battery charge/discharge current from a Hall CT), I generally lift the waveform up so that it remains positive at all times. Here, measurement is based on the DC offset.


"But in case I want to retain the waveform and make sure that zero crossing remains zero crossing, how do I insert a DC component without having to tune the waveform offset for every card? "

If I have an ac wave to be measured, then I generally use precision rectifier, or I just rectify it and manage the error through calibration. Here, if I add a dc offset, then the waveform will definitely go up but wave will not be aligned to zero axis.

Or, did you mean to clamp the wave using an opamp, and add the offset=0.7 at the opamp and then take to the ADC? This will not work.