How can I lock my door?

I just moved into my new house (college student) and I have a problem with my bedroom door. Apparently there's no holes drilled for modern day doorknobs. The house was built in the 1890's and lock/doorknob look like it. The key hole for the seized up lock looks like a "pirate key." I have a chain lock from the inside of my room to lock the door but when I leave my room, I want a way to lock it from the outside. For example when we throw a party, I don't want people in my room. So is there a way I can lock my door from the outside?