Ok I got this going, the full howto would take me forever to type it up, so here's the method I used, hope it'll help someone out.
Note: I couldn't get the admin mode working, whether I missed something I don't know, so I'm sure there will be an easier way than this! If anyone has it please let me know.
Create a new user on another linux machine with Firefox installed
Add desired extensions
Extract Porteus Kiosk ISO
Unsquash the settings file
Copy the existing Porteus Firefox profile somewhere for comparison
Replace the Porteus Firefox profile with that of the new linux user
Use Meld to compare prefs.js between both and amend the Porteus one accordingly
At this point it should work but will likely complain about missing extensions due to extensions.sqlite being different
Install SQL Lite manager on development machine (Firefox addon)
Examine the backed up extensions.sqlite file and make a note of the addon details (I exported them to a csv using the addon)
Open the Porteus user's extensions.sqlite and ensure the addons table includes the new addons
Recreate squashfs
Recreate ISO
All done, working great with touchscreen style keyboard addon.
#g.