How do I change the contact form in Upfront?

I need to edit the contact form, but in Upfront there does not appear to be any way of changing anything but the labels on the existing form (which is a bit buggy, as in the editor it reverts to the original on the screen, even though it does actually update the page on Save).

Apart from adding a telephone number field, I need to control the messages, as the site will be in three different languages. I would also like the flexibility of a contact form plugin like Contact Form 7 - especially adding Re-Captcha - but I can't even access your own Contact Form Widget plugin on Upfront-based sites.

Is this a development issue or is there a way round this?