I'd like to add a glowmap I made to a mesh, but I can't figure out how. The texture I want to applied to lists options such as "Add Dark Map" "Add Decal Map" in nifskope but not glow map. For other textures, "add glowmap" sometimes is an option.

Why is it sometimes an option, and sometimes not?

It's not necessary to add the glow map directly to the mesh. Your mesh will glow in the desired ares as long as you set the Emissive Color of the part you want to glow to white (R:1 G:1 B:1 in Red/Green/Blue format or #ffffff in hex). Setting it to white ensures that the glow will be the same color as the texture applied to the mesh. You can set this by expanding the NiTexturingProperty node, then selecting NiMaterialProperty. You'll find the Emissive Color setting in the Block Details list.

Note: In order for a glow map to function in the above manner, the glow map texture must have the same name as the base texture, followed by _g. Example:

TextureName.dds ; the base texture you're working with.
TextureName_g.dds ; the glow map for the texture.

Thank you sir.