Grease pan?
Not sure what you mean. On the 3/5/700 there is no pan, there is a drain, that drains off the grease. The grease collects in the bottom and funnels off. Funnel is at the front of the 300 (actual it's a ballcock open/close)
Did CS make a change (the manual doesn't reference a pan).
I've cooked on a 300, but didn't have to clean it.
It's pretty normal, people want to come in an change how CS designed it because they used a certain method previously (like a water pan). It's a pretty humid smoker (depending on load) so my suggestion would be to use it a couple of times as is, figure out times/temps.
I'm not a big fan of water pans, I don't think it adds to the moisture of the meat, it tends to soften the bark a bit.
But if you're still set on it, I'm not sure how you can add moisture (I don't know of the 300 pan) but you could add a tray on one of the rotisserie shelves.