Looking at the
transmissions of a typical DSLR CCD sensor with a Bayer matrix the Ha will only go to the red channel while the O-III will only go to the green and blue channels.
So it should be possible to split up the RGB channels and get Ha O-III narrowband images with Ha in the Red channel only and then add the GGB channels to get the O-III data only.
Of course there are tradeoffs compared to the real narrowband:
1. the resolution will be 1/4 of the full sensor resolution.
2. the efficiency will suffer.
Only a 1/4 of pixels for Ha are used and unmodified DSLRs have a poor Ha transmission rate already.
It looks better for O-III though, with 3/4 pixels actually being used at about 50% transmission rate for each pixel (dependent on the Bayer matrix, the figure of 50% comes from the example transmission rates in the linked image).