Yeah, so typically you get something like a 2 beat fill put in a 4 beat (1 bar) MIDI clip, with the first couple beats being foot hats. You’d need to typically fit that after a shortened bar as well, so drop it in on top of what you have, shorten the preceeding (non-fill) clip from the end, and shorten the fill MIDI clip from the start, making them meet as necessary to sound natural. If you still have an undesirable hihat beat in there, you’d need to open the MIDI editor and delete it.
I’m just going to go ahead and say what needs to be said. This is really, really basic MIDI editing, so you should, if you are going to be working with MIDI, find a decent Cubase-specific MIDI editing tutorial. Cubase 6 comes with some (12 to be exact), but the files don’t seem to be named very well. I would start there and move on to Bingle (i.e. Bing/Google) after that.
You need to know how to manually edit MIDI to work with AD and Toontrack drums. You just do.
EDIT: Never mind on the included tutorials, at least they don’t seem to have anything MIDI-editing specific. Looky here.