It is not possible to create fields that are specific to the Collector for ArcGIS application. The fields must show up in the source feature class or ArcGIS Online hosted feature service. The nullable and/or non-nullable property between the Collector app and its source service cannot be changed as the Collector app is simply reading the source service.
The Collector app populates the editing template with the feature template from the service, and behaves the same whether it is an ArcGIS Server feature service or an ArcGIS Online hosted feature service. The Collector app does not allow users to submit an edit with a null in a non-nullable field. Even if it did submit, the feature service rejects the edit accordingly.
When publishing services to ArcGIS Server or ArcGIS Online, ArcMap will enter a space " " for non-nullable text fields and "0" for non-nullable integer fields, therefore the fields are not technically null. When publishing ArcGIS Online hosted feature services directly from a zipped shapefile or file geodatabase, if there is a non-nullable field, it holds the space " " as an edit, so that the nun-nullable field has a value and does not submit a null.