Here is one of several ways to add Ha data to an RGB image with PhotoShop. This works best with emission nebulae. With care, it is possible to add the Ha data without destroying any reflection nebulae. Additional processing steps may be required to use this method with a galaxy.

Process the Ha data to achieve maximum detail and high contrast.

Now make an RGB image with only the HA data in the red channel. Here is one way to do it.

Convert the Ha image to RGB. For the green and blue channels use "Select All", then delete the data and use black for filling the contents. This should give you an all red image with the green and blue channels set to black.

Paste the "all red" image onto the original RGB image as a new layer and set the blend mode to lighten..

Finally, use the Levels Tool to adjust the visibility of the top layer to achieve the desired effect.

Run your mouse over the above image to see the before and after image.