At the hardware store or home center you can buy clear plastic tubing that is very flexible. You will need about 4 feet approximately 5/8" to 3/4" O.D about 1/16” wall. Also need a funnel that will mate with the tubing. From the back of the engine compartment, snake the tubing down toward the fill plug of the gearbox. Using the funnel connected to the tubing pour from the engine compartment side. With the car level and the transmission up to temperature the correct level is so that it just runs out of the fill port. Check the Bentley manual for details of temperature and shift lever position.
Warren
91 318is
95 525iT