add subs to integration

There is yes, just load in the integrated lights and add the new data as well. I would first get enough extra data and calibration data to make an integration of that and then combine them.

@vincent-mod

If I may expand on this a bit: On M31 I have Ha, L, R, G, and B integrations I am happy with qualty-wise. However the total times could be higher, especially the L data.

Next year could could I shoot more Lights, calibrate them, and the add the new L channel integration to the old L intgration?

If so, would this be done in the Integration tab, or the RGB Combine tool? Presumably I would use the Registration tab 4 to register New to Old.

Conceptually then the Integration file is a Master Light. This would imply good news for my disk management as I could delete individual subs once I have good integration files, in the same fashion as Master Darks, Bias, and Flats. 

You can yes. Given your integrations are calibrated well with enough calibration frames, you should have clean integrations with good signal so those will add nicely when stacked. Theoretically it's better to stack all subframes, but personally I haven't noticed a massive difference in that sense. I would test it for yourself first as well.

You simply load your integrations as lights, without calibration data and integrate.