I haven't been to the Warmoth website for a while.... But originally, before they offered the Custom Builder app within their ordering process, they had a webpage which detailed the optional extras.... You then had to email them & get a quote to get your order started....
There ARE limitations to what they can and cannot do and sometimes the 'Nos' can seem quirky, but they have their reasons. It's always good to ask if they CAN do something first, before committing to hardware and everything else on a project....