If she is not married, she's fair game. Tell her "Hey, I doing (make up some planned activity like a party or something) this on Saturday. It'll be fun. You should come." If she says "I have a boyfriend", just say " I'm not asking him I'm asking you". If she says no, fine but don't give up. Ask again for a different activity, but sound like it is no big deal if she says no. Just let her think you will have and had a great time(after the weekend) by yourself. I'll bet she says yes before long. Just don't act too interested in her and act like she needs to impress you.
Oh, and BTW I have many Japanese ladyfriends who I never dated.
