[Rules] Activating units outside of an Assault to move on a IF

[richvalle]

4th Ed.

This one is new to me. 

Leader inside an assault with 3 friendly units, one of which is Dem. He activates all 3 in the assault hex and 1 unit outside of it to move in IF the Dem unit fails to recover. Is this valid?

So he's looking to try and recover, if succeeds he assaults with the other 2 units, if recover fails it flees and he brings in the outside unit and assaults with 3 units. 

My first reaction is to say 'no' but I can't say for sure that is right. Whether the Dem unit recovers or not the outside unit is going to be 'Move/Fire'ed.