Yes it is possible and depends on operator. because no one restricts the IMS signalling in particular bearers. QCI 5 is having higher priority so that IMS signalling will be reliable but if you don't have QCI 5 still you can transfer IMS signalling on any bearers if operator don't have any restriction policy. Mainly restriction comes to maintain quality of service, because assume you are transferring IMS signalling in some other bearer and it dropped because of some reason then user will face bad experience this is why they use only QCI 5 so that it will not drop.
Note : No 3GPP spec says restrict particular data or signalling on particular bearers. It is up-to operator configuration.