Static Extensions in Deviceanduser mode

Is there a way to have a bit of a mix and match approach to deviceanduser mode?

We have about 100 handsets and about 50 want the option to hot desk, so need to be able to log on to a phone as they move around. However the other half won’t ever move from their desks (some people haven’t moved in 10 years, I swear), and these people ideally need to just have their extension assigned.

Is it possible to create some fixed extensions? I thought I saw something about it in the forums but I can’t find it now.