No, it's not possible to pre-populate a password field. Since it's starred out ****, browsers won't let that be pre-filled with a value. That's just how browsers work since there'd be now way for the user to know what value is pre-set there for them.
On our JFBConnect registration page, we have the ability to hide the password field and generate one automatically when the registration form is submitted. We don't have that level of control over the JomSocial registration page though.
I hope that helps explain,
Alex