A scripts or a very complicated parallel process by the side would be needed. See rpg maker uses the character sheet for movement and like you said there is no graphic difference between normal and sprint. Since you have made a custom graphic, please make sure it is the right format like the defaults.
Not sure about the script but a parallel process could work by checking if the "Shift" key is being pressed and if so changes the character graphic to the sprint version. To do so, make a new event somewhere out of the way on the map. Make it a parallel process and below characters. In its contents have a conditional branch = "shift" key is pressed and if true add in the true branch Move command -> Player -> Change graphic to sprint version. In the false branch add also a Move command -> Player -> Change graphic back to normal. After the conditional branch, not in any of its branches, add a wait command of between 3-30 frames. If it doesn't seem to loop, put all the contents in a loop command.
This should work like this:
1. Player walks with normal graphic
2. Player presses the shift button, satisfying the condition and changes the player graphic to the sprint version.
3. Player lets go of the shift button, player graphic changes back to normal.
