Tho I haven't done this personally, I know that it was done a lot by our predecessors. Bonnie Hunter just showed a quilt on her blog that had worn spots where you could see it was made over another quilt underneath. That was probably done when old quilt fabrics wore out but the batting was still good.
I have to say tho, if your quilt is in good shape it seems a shame to use it for batting. If it were me, I'd donate the quilt and buy new batting for a new quilt instead. Someone, somewhere, needs that quilt and will love getting it.