any public variable in any script can be exposed in Playmaker by dragging the component from the inspector into Playmaker's state panel, where you'll get a prompt asking you whether you want to set or get a specific variable. In your case, just use SetProperty and the velocity should appear in the dropdown.
you cannot instantiate a new vector3, except if you use hashtables with ArrayMaker.
The action TimeInfo can give you deltaTime. then use float operator or float multiply to get a float that fgits your Y coordinates. Then create a vector3 and use a set vector3 XYZ action to make it 0/float/0.