In my findings is yes, you can refill through the dip stick tube. The below is from Just answers
You can fill your Volvo Penta drive unit by removing the dipstick from the top of the drive unit, remove the front starboard corner fill plug on top of the drive unit (allen headed screw with a small oring underneath it - use caution not to lose the oring, replace the oring if damaged), then use a small funnel to add the drive lube into the fill plug hole (add 2 and half quarts), let the lube in the drive settle for a few minutes, recheck the lube level on the dipstick and top off as needed.
Alternatively, you can refill the drive unit by removing the rear cover of the upper drive housing, once the rear cover is removed, you will see a vent screw (typically an allen headed screw) on the rear starboard side of the upper drive unit, remove the vent screw. Fill from the bottom drain plug until lube runs out of the vent screw, reinstall/secure the vent screw and gasket, let the lube in the drive unit settle for a few minutes, check the lube level on the dipstick and pump more lube into the drive (from the bottom drain plug up) until the correct lube level is reached on the dipstick.
Yes, it is also permissible to remove the dipstick plug from the upper drive unit, then fill from the bottom drain plug hole until the correct level is obtained on the dipstick. This method will require patience as you will have to pump the lube into the drive and continually check the dipstick until the correct level is reached on the dipstick. Its not uncommon to have no lube showing on the dipstick until the lube is allowed to settle inside the drive for several minutes.
After filling the drive unit, be sure to let the lube in the drive settle for a few minutes or longer, recheck the lube level on the dipstick and top off the lube as needed (or remove lube if overfilled).