Hello Shydinky

Welcome to PCHF
From the sounds of it, you are not going to be able to add the Poseidon expansion to the original Zeus game. Your best bet would be to purchase another copy of the original game and make sure it's the right version.
As far as some sort of patch or workaround, the only place you can go for that answer is the company itself. I would contact them directly through e-mail and explain the situation and see if they have any info for you. Make sure you include any error messages word for word, including punctuation, that you encounter.
But since the UK version isn't working, that means there is a difference in the programming somewhere. I would expect you would have to have the UK expansion to go with the UK original. I doubt they have some kind of UK to US patch or something of the like.
Hope that helps!
