If the volumes are replicating you will need to stop the replication task. I believe in the future engineers are looking into this feature (not ETA!!) but the process would be that you would have to increase the Destination volume first then the Source for obvious reasons but for now you have to stop both and increase the size for both (being equal on both ends of course).

"" Or is it possible to add/remove replicated volumes without disturbing replicating volumes that are in use?""

With manual failover yes but if you need to perform this in Auto Failover no.