If possession is nine tenths of the law, Richard Simmons will soon call Barbra Streisand his own. The fitness guru and Streisand devotee has hired doll artist Lisa Lichtenfels to create a 30-inch, $10,000 diva just for him. ''Richard wanted her to look like she did on tour wearing that black dress,'' says Lichtenfels, who assures the replica will be accurate, down to its ''press-ons for her fingertips.'' The details have Simmons who will first see his beautiful doll on Oct. 6, at the International Doll Art Show in Chicago swooning. ''Having a doll of Barbra is probably the closest I'll come to being alone with her,'' he sighs. Streisand herself had no comment on the soft sculpture, but a spokesman couldn't help but laugh: ''Richard's always doing such interesting things.''