I have another way to do this:
1. Create the base drum pattern for a musical phrase, lets say it's 4 measures long. You will end up with 100% velocity for all notes
2. Switch from the Draw tool (d) to the Select Tool (s)
3. Draw a select box around just the hi-hat notes you want to tweak, in those 4 measures. Those notes - and only those notes - should be highlighted when you're done
4. Go to the Velocity setting in the MIDI toolbar, and change it to, say 75%
5. Play it (space_bar) and see if you like it. If not, the hi-hat notes should still be selected, just change it to 65%, etc...
6. Tweak the velocity of the other drum parts until you're happy with the phrase
NOW:
7. Copy the phrase over 4, 8, 16, 32, etc., times, until you've filled out that section of the song. All your velocity customizations will copied along with it
8. Manually make the phrase or section-specific customizations required, e.g. put in drum fills and the end of phrases, and transitions at the end of sections
This works well for me and saves time, as I can keep one hand on the keyboard and one on the mouse - no hunting and pecking in menus or dialog boxes...