I'm operating mostly under the function that an add-on can be treated like a mod. Which is to say that it is within our scope to a certain extent, what determines the full account is what kind of question it actually is. If I'm wrong, then it just chalks up another point to the fact that, like "mod", "add-on" is horrendously useless as a general term. So my thoughts are:
If the question about the add-on is more about development principles or quirks that are used in modifying them, then it would probably belong on Game Development, under the assumption that they are "mod development" type questions. (This is being questioned with some Game Dev folk, so don't get too hasty on migration flags)
If the question about the add-on is more on the vein of doing configuration settings to get it to work or within the predefined parameters, then it would be acceptable here. But all the same, like with mods, keep in mind that the developers of the add-on typically have better documentation than we will.