The short answer is yes you can. These transmissions sometimes leak and the leak will need to be repaired and the fluid put back into the unit.
There are normally 4 or 5 bolts that hold the bottom cover on the transmission. Those will need to be removed and gently remove the cover and empty the transmission fluid. Next, you want to clean the cover and the transmission mating areas of any dirt, debris and fluid.
Now you want to replace the gasket if it has one and fill the bottom cover to nearly the top with 10W-30 or whichever transmission fluid your lawn mower calls for. If you have your mower on a lift, this make the job of putting the bottom cover on the transmission much easier.
Install the bottom cover and start all of the screws and then tighten them up sequentially so that the cover does not tip and spill the fluid out all over the place.
Then as they get tight, you just want to snug them up and don’t over torque them because they will strip out easily. If you used any sealant on the cover, let that dry for a day and then test drive the lawn mower to be sure there are not any leaks.