How customisable is CoffeeCup, could I perhaps create new HTML button types or field types?

The reason for this is I would like to use Coffee Cup for designing Forms used on another site, this site has additional 'HTML' field types. To be able to use these additional field types would help in the development i need to do.

When you say 'Coffee Cup' you are talking of a handful of different programs. You can design forms, buttons and form fields in Site Designer, HTML Editor, and Form Designer, but you need the Web Form Builder in order to create the necessary back-end script to make the form work - unless you are PHP savvy and can create your own back-end script.

Hi I mean is it possible to add the following into coffeecup Form Designer

<button type="roll" value="/roll 1d20 + @{Bluff}" name="roll_BluffCheck"></button>

Yes, you can add custom html to make this work.