You could also start using it right now. For endpoints which are already implementing JSON:API go with the defaults. If the endpoint is already RESTful customize Adapter and Serializer to match the current implementation. If it’s not yet RESTful, you may be still able to customize the Adapter and Serializer to match it. There is not restriction preventing you, to implement a Adapter which is using POST to fetch records. If this doesn’t work out for some endpoints, you could use fetch and push the record to store manually - or if you don’t need them in store (e.g. they don’t have a relationship to any other object in store), just go with it. This would allow you a smooth transition.
Also have in mind that endpoints which do not represent entities but actions for example, fetch is mostly a better solution than fighting ember-data. Also have a look at this discussion.