You can easily drain the pan and converter from underneath. I would not mess with the topsider for this. Unlike the oil pan in your engine where nothing is in the way, There are all sorts of things in the pan of your A/T that might be bumped around with the topsider. Since you have to get underneath for the torque converter anyway, use a drain pan or two. You can drain the pan, empty your drain pan, then drain the converter.
Okay for the main reason I answered you post: DO NOT USE THE SAME FUNNEL TO FILL YOUR A/T THAT YOU USE FOR ANTIFREEZE. THE SMALLEST TRACE OF ANTIFREEZE THAT MAY BE LEFT ON THE FUNNEL, EVEN AFTER WIPED OFF THOROUGHLY, CAN KILL YOUR A/T CLUTCHES.
I hope you got that, anti-freeze is as poison to your A/T as it is to Fido.