I'd probably go about it this way, editing obj_player a bit:
Add a new variable iscrouching which defaults to false, is set to true in the step event whenever the crouching key is held (you'll need to edit the get_keys script to add in checks for the new crouch key) and false otherwise.
Add in a new code block in the Begin Step event: when the crouching variable is true, lower zheight, movespeedmax and jumpspeed to something lower than their defaults, when it's false set them back to their default values. zheight in particular influences both the player's collision checking and where the 3D camera is located.
If you want the camera position to smoothly move up and down, you could use the lerp function rather than setting the zheight variable directly to the values for crouching/standing.
In order to avoid collision issues where the player crouches, gets under something low, and then un-crouches, you'll probably want to only allow switching from crouching to standing when there's enough free space over you that you can stand there with your original zheight. Depending on how you design your levels, though, this might not be a problem (e.g. if the player never crawls under anything and all ceilings are so high you can't bop your head into them, they can't get into a situation where this collision thing becomes an issue)
Let me know if something's unclear, and good luck :)