can you make up your own martial arts?

Martial arts began by people creating their own martial arts. Someone invented Karate from the ground up same with Kung-fu, Taekwondo, and all the others. The ideas in this world are limitless therefore so are the martial styles and arts. You can definately make your own martial art that is what Bruce Lee's Jeet Kun Do teaches us.

Everyone uses their own martial art... so the answer is yes.