Why is it so hard to add a "Add Layer" button to an existing hosted feature Layer in the content data section in AGOL, right beside the + Add Field button? Am I the only one who wants this functionality?
ArcGIS Ideas is the place to suggest your idea.
You can overwrite your layer from ArcGIS Pro with a new layer added, using the ribbon button (taking into consideration the issues that can occur with overwriting any layer)
Controlling this action from ArcGIS Pro ensures that the person who published the layer and owns the source data with permissions can make these structural changes.
You can also add another layer to the map in ArcGIS Online and group them.
Is there a specific reason this would be helpful for you?