There are several ways..
I would just grab the current X,Y (and angle unless its always facing one way as in a side scroller) position of the player somewhere in its transforming state and then use an event that triggers at the appropriate time in the transition and spawns the new sprite at that time at those coordinates.
You could also fire an event off the player sprite that uses action [Spawn another object] and choose the appropriate image point for where it come in (usually center 0). You could pop it in at end of transition, start it off invisible and then change to visible at end of transition, or start at opacity 0 and slowly fade it in by incremental opacity changes..
it really depends on the look your going for.
In my space shooter linked in my signature below all the available weapons are always attached to the player from the start of the game. I simply turn them visible when they are activated and back invisible when deactivated. That said, you could have all the available sprites pinned to your player with position and angle from start and simply set the ones you want visible at the appropriate time while setting the Player/last transition to invisible.