WARHAMMER 40,000 RULEBOOK Official Update Version 1.6 wrote:Q: When a rule allows a model or unit to take an action (move, shoot, charge, fight or attempt to manifest a psychic power) outside of the normal turn sequence, and that rule explicitly mentions to make that action as if it were a different phase of the turn structure to the current one, e.g. ‘That unit can shoot as if it were the Shooting phase’, do rules that are normally used during that phase (in the example this would be the Shooting phase) take effect? Is the same true of Overwatch attacks?
A: With the exception of Stratagems, all rules (e.g. abilities, Warlord Traits, psychic powers etc.) that would apply in a specific phase apply to actions that are taking place ‘as if it were that phase’. However, if a Stratagem specifies that it must be used in a specific phase, then it can only be used in that phase (e.g. you cannot use a Stratagem that says ‘Use this Stratagem in the Shooting phase’ to affect a unit that is Shooting ‘as if it were the Shooting phase’). For the purposes of this FAQ, Overwatch attacks are not considered to be attacks made as if it were your Shooting phase.
So no, you can't use stratagems unless the stratagem is usable in that phase or the stratagem has no phase restriction.