Adding Words to class lists (wordspells)

[Portilis]

I've come across an issue that I'm currently trying to sort out, and I'm wondering if anyone has any thoughts on the issue.

I'm playing a Wordcaster Witch in a campaign with a shamanic basis, and I need to work out how to add some extra Words to my list from the existing words for other classes, as I have Eldritch Heritage (Arcane) and will be getting New Arcana later to get a few extra elemental spells from the wizard list.

Now, testing with that and the Samsarran Mystic Past Life ability (to see if that worked differently), I can add a word from another list to mine, and it is available to pick up in the spellbook, but having added it to the spellbook, when I go to create a memorised wordspell the additional word is not on the list to choose from.

I've had a careful look through to try to work out whether I can get around it by adding new versions of the wordspells directly through a customised version of the feat, but they appear to be in the background somewhere rather than being possible to add through the editor, so I hit a dead end there.

Any assistance would be appreciated!