To just combine (aligned) L with more L-frames, I'd use ImageIntegration.
To combine L with Ha, for instance, if you want to create a new channel, PixelMath is the one to use.
If you just wanted to combine a set of 3 or 4 channels, you could use LRGBCombination if all you want to do is change (or not) their individual weights.
For anything fancier, as in combining two frames in one channel with their own weight, then PixelMath is the one I'd use.