I'm presuming that the staircase is the only method of access to and from the units. Two things to consider, is there space for a chair and or table without impeding access to or escape from the units in the event of an emergency?
Are you adding to any combustible material in the stairwell? – in the case of wooden, plastic or upholstered items you certainly would be.
In the case of office buildings typically there is no material in stairwells which is combustible, carpets are allowed on stairs in residential buildings to reduce noise, and of course older ones may have wooden stairs which wouldn't comply with modern building standards.
You should be having an OH&S inspection every year, suggest you ask the inspector about requirements before adding any furnishings to stairwells.