Nobody can say anything. We know nothing about how your footage actually looks, what version of AE you use, what system and so on. that aside, I smell a fundamental lack of the meaning of Add blending mode here and how it actually works. They may promise that on commercials for washing agents, but you can't have "whiter than white" in compositing. Just because the white disappears doesn't mean there is genuine transparency and that's where your whole plan falls apart. You need to create genuine transparency by unmultiplying whatever you have (on a black background, no less) or constructing it with proper tranbsparency in the first place...
You will have to color correct the white robe so that is is dark enough to see the brightest part of the light sobers glow. Simple as that. There is no brighter than white unless you are working in float. Change the project to 32 bit color, apply levels to the footage to bring down the whites, use the add blend mode. That's about all you can do unless you can post a frame of your shot so we can come up with other ideas.
I will also suggest you do more homework and prepare to reshoot some of your footage.
There are literally tens of thousands of Star Wars fan videos on the interwebs and some of the better clips have how-to stories and help files. You can also find extensive, professional-grade tutorials for light saber effects on many After Effects tutorial sites. While it phases in and out of popularity, creating light saber duels a very hot topic.
Try to have fun. If you shoot your video properly, AE will make it happen for you but it's not magic. There are fundamental technical limits and specifications AE requires.