What kind of pump? Hand or electric? Can you cut the "big thing" off the end of the pump hose and keep proper pump functionality? Maybe the thing is a check valve to prevent backflow into the pump, which is not critical for this situation.

Ideally you want a hand suction pump that looks kind of like a grease gun. You pull back the plunger to fill from bottle, then put hose in upper fill hole and push the plunger.

In a pinch, sometimes you can use an empty, clean rear axle fluid bottle. The kind that comes with a pointed tip that you cut to desired size. Get a length of 3/8" I.D. hose (about 2 feet). Cut the tip from the axle fluid bottle so that the 3/8 hose fits tightly. Cut the hose to the shortest length that will allow you to manipulate the bottle to a point higher than the fill hole, under the truck. Fill the axle fluid bottle with your trans fluid and attach cap/ hose assembly. With the hose in the hole, begin to squeeze or crush the axle fluid bottle to fill the trans etc. When the container is empty, you can reshape and refill it several times, but watch for leaks.