You can add any custom keybindings you like to the Symex modal UI by using the underlying Lithium facilities for this purpose, as described in the Lithium documentation.
PLEASE NOTE: if you use custom keybindings, it is essential to preserve the intended “exiting” behavior of the commands. For instance, all insertion commands in Symex are designated with :exit in the keybinding specification, which ensures that it exits the Lithium mode to allow you to enter text. If you choose to override these, remember to retain the :exit flag (described further in the Lithium documentation).
Also See Point-Free Modal Editing for more on Lithium.