You have to place them individually. You can speed the operation somewhat by placing a group, and then doing a copy and paste of the group.
Be aware that from a design standpoint, the random placing of vias (sometimes called peppering) may actually cause problems rather than solving them. The spacing of the vias can end up giving rise to resonant loops that become secondary emitters in RF circuitry. They can also interfer with the return path of critical signals on the plane by introducing holes around which the return current must flow in a longer loop pattern. The increased signal loop size can give rise to unexpected coupling between signals, ground bounce, and other undesired results.
The best way to place connecting vias between planes in critical circuits is to model them first in a good 3D simulation program. Don't pepper vias just for appearance sake - make sure there's a good design reason first.