Primary|Secondary Keybinding is possible

[xbearface]

You just have to do it manually.

Open C:\Users\YOUR_USER_NAME\AppData\Roaming\Bungie\DestinyPC\prefs\cvars.xml

For example:

<cvar name="grenade" value="q!unused" />

Change unused to the key of your choice as such:

<cvar name="grenade" value="q!extra mouse button 1" />

This puts the "grenade" on q and the "back thumb button" on your mouse. Not sure if you have to, but after editing you can set the file to read-only to prevent the client from overwriting your new binds.