I wanted two actions to remap: closing the conversation dialog with Escape key and show/hide offline buddies with Control-H.
This can be achieved by editing the ".purple/accels" file, which is located in your home directory on Linux and in "{drive letter}:\Documents and Settings\{user name}\Application Data" on Windows, and inserting the following lines:
(gtk_accel_path "<PurpleMain>/Buddies/Show Offline Buddies" "<Control>h")
(gtk_accel_path "<main>/Conversation/Close" "Escape")