You can if you use Ember.Handlebars.registerHelper which bring different function parameters. Once, you get the container you could look for any registered instance like your session.
I have not tested, but I think something similar to this example must work:
import {handlebarsGet} from "ember-handlebars/ext";
registerHelper('formatDate', function(value, options) {
var container = options.data.keywords.controller.container;
var session = container.lookup('app:session');
var propertyValue;
if ( options.types[0] !== 'STRING' ) {
var context = (options.contexts && options.contexts.length) ? options.contexts[0] : this;
propertyValue = handlebarsGet(context, value, options);
} else {
propertyValue = value;
}
return moment.unix(propertyValue).format(session.dateFormat);
});
Take in consideration that helpers created with this method will not re-render their content when data changes. If you need to define a "bound helper", take a look at Ember Handlebars Helpers.