Technically, the monitor nozzle is the part where the foam induction & aeration (in case of primary aspirated foam) takes place. Hence it is possible to convert a water monitor to a foam monitor by changing the water nozzle to a foam nozzle (aspirated or unaspirated).
However, it is important to ensure that the capacity of the foam nozzle is matching to the capacity of the water nozzle to get required performance. Contact the monitor supplier to check suitability. Note : Connecting a Foam branch e.g. FB 10X (450 lpm capacity), at the outlet of a 75 mm waterway monitor (which typically has a flow of around 2800 lpm) will produce only a limited foam range i.e. the range of the FB10X branch only. Such arrangements are not recommended.