Rule 213.5 appears to allow the player to Build-Up at any point during the move. For example, a Panzer Corp can move on top of another Panzer Corp, and they can then form a Panzer army. In TitE, only prior to Movement was this allowed. In ETO, there is no rule stating prior to movement (or the movement of the units).
Is that the intent? (Voluntary Breakdown specifically states prior to movement, if it matters).