Yes, it is possible, although some things might be easier if the layer was added to the map view.
If you don't add it to the map, you will probably need to instantiate the feature layer in either snapshot mode, or selection mode. This doc, ArcGIS Feature layer , contains some information about the different modes.(In contrast, the OnDemand mode is better suited for when you add the layer to the map, in which case the layer will retrieve features from the service that fall within the map's view extent.)
Without adding the layer to the map, the features won't display, and users won't be able to tap on a feature to begin editing. Your app will need to handle this workflow a different way or write some code to provide this experience.
To edit a specific feature, you will first need to query it using [AGSFeatureLayer queryFeatures:]. Once you have a reference to the feature, you can modify its attributes/attachments/geometry, and then send the updates back to the service using [AGSFeatureLayer applyEditsWithFeaturesToAdd:toUpdate:toDelete: ]
This sample (RelatedRecordEditingSample) uses this pattern to edit records (incident notes) that are related to a feature layer (incidents). The incident notes are pure tabular data and cannot be visualized on a map, hence the feature layer that is used to edit them is not added to a map. (See NotesViewController.m )
