Modules are responsible for presenting service related fields during checkout, so this would be a question for the author of the NOC-PS module. For other types of fields (checkbox, radio, drop down, quantity), these can be added as configurable options.
It sounds as if a hostname field is something that should be standard when ordering a dedicated server via NOC-PS, no? I'm assuming it supports the ability to set the hostname for the server, so it would make sense for it to ask for it.