Hi-We have a 2 story house with the masterbedroom over the garage in Michigan. The bedroom gets very cold and we are trying to figure out ways to make it warmer. We have a forced air furnace with the ductwork coming in from the ceiling (strange I know). We also have our cold air return abour 1 foot from the ceiling. It seems like it is pulling out the warm air due to it has fairly good pull when the furnace is on (I lit a match and it would pull it with force). Can I simply add another grate directly below it closer to the floor, while closing up the old one, without screwing anything up?

Yes and no.
First make sure the garage ceiling is insulated and dry walled. This is necessary for the insulation purpose but also for fire code since a habitable room is above the hazard area.

If the wall is open from the one grate to the opening you are going to make below you sure can since you will be using the wall cavity to convey the return air back to the furnace/air handler. Make sure you do not reduce the size of the grill opening during this procedure and I would also cover the existing hole tightly BUT temporary since you will want to see how the A/C reacts during the summer with the change of return air location. Cold air falls so the high return might be working well for the A/C so in that case you will need to change over the return air every year to suit the seasons. I have a similar situation here and use a magnetic cover to close off which ever return I want during the seasonal change.