As the new Fire Tank addon is just a retexture of the fuel tank, the Paystar Twinsteer, ZiKZ 605R and DAN 96320, due to having custom variants of the normal fuel tank, currently do not have the option of a Fire Tank addon.
The Paystar Twinsteer has a unique extended variant, while both ZiKZ 605R and Dan 96320 have an extended frame underneath them (unique for each of them), which are the reason of the Fire Tank addon not being available for neither of them.
Previously reported by Nextej at https://forums.focus-entmt.com/topic/64308/water-tanks-feedback.