Another way is to crack open the .sublime-commands files.
Let's say you've installed Sublime Package Control (which you really want to do!) and then open it up in the command palette (⌘⇧p on os x) and install the Search Stack Overflow package. You'll now have two new commands in the command palette, the "Stackoverflow: Search Selection" and "Stackoverflow: Search from Input" commands.
OK, open the .sublime-commands file for the package. You need to find it first. If you're hardcore you do View > Show Console, and enter print(sublime.packages_path())
Otherwise it should be here
- Windows: %APPDATA%\Sublime Text 2\Packages
- OS X: ~/Library/Application Support/Sublime Text 2/Packages
- Linux: ~/.Sublime Text 2/Packages
- Portable Installation: Sublime Text 2/Data/Packages
and then "Search Stack Overflow/Default.sublime-commands"
This is the file that make the commands show up in the command palette in the first place.
It's another JSON file with entries like these
{
"caption": "Stackoverflow: Search from Input",
"command": "stackoverflow_search_from_input"
}
see, that's the command name right there: stackoverflow_search_from_input
Now just open the user key bindings JSON file and add the key binding like @BoundinCode said.