When I first started formal FMA training in Latosa PMAS Escrima, I guess it was sort of an add-on to my core system at the time: Wing Tsun. I continued training and got deeper into Escrima, and even taught the art, but mostly with WT circles. Later, I got out of the martial arts completely for a long time... about 16 years. When I started training again it was the FMAs that pulled me back. Soon I was back at my old core art too, but this time under the name "Ving Tsun".
Now I see each art as complete, yet compatible, since I now aproach them more from a conceptual perspective. At longer range, when I can get any improvised weapon in my hand, what I do looks like escrima. Closer in, and empty-handed, it looks like Ving Tsun. But under the surface, I'm applying the same fighting concepts in the way that works best for me. And, best of all, my instructors in both systems are cool with that.
So for me, neither art is an "add-on", and though each system is complete by itself, I find that without trying to make some kind of "chop suey blend" nevertheless, each one has strengthened the other for me. In other words, they are neither separate nor blended, but different expressions of the same concepts of combat.