Hi Adrian,
I have even taken a fully-processed RGB image (from an OSC source) and 'added' it to a fully-processed Lum image simply by using the DynamicAlignment process to register one image against the other - and then to use the LRGBCombination tool to add the whole lot into one new image.
Don't worry - what you seem to be trying to do has been tackled regularly by other PixInsight users. However, that said, if you do have problems, you should be able to get help here on the forum.