if the system kicks on, and stays on (check the compressor clutch pulley and have someone hit the button on and off) the possiblity of topping it off is different than if it shuts down.

R-134a is more forgiving with pressure variance than r12 was. you can slightly over the charge as system to adjust how cool it gets without immediately causing harm. IF the system stays on, you will be able to give it a slight charge out of one of those cans with the fill button. But i would not add too much. I top off systems occasionally and rarely run into trouble, just watch the vents have the system on high and stop when its a comfortable cool and the gauge on the bottle is still in the safe range.

disclaimer. topping off your system will not fix a broken system or leaks, and if you do not need a charge added can cause further damage if there is another issue.