Thanks Jarens, I got that bit and going after the addSelectOption route, but I was just curious whether the amount of values you add for each line using addSelectOption could be different per line. I'm guessing this is not possible and once you build your list it will be spread across all lines.

I am doing this in many of my Suitelets and all you need to do is use addSelectOptions under if conditions. E.g. If(firstname == 'John'){ field.addSelectOptions('random option')} else if (firstname == 'Doe'){field.addSelectOptions(more random options')}