Can sub-layers be added to an existing hosted feature layer via the ArcGIS Online web interface?

Answer

No, sub-layers cannot be added to an existing hosted feature layer via the ArcGIS Online web interface. In ArcGIS Online, the Merge Layers tool can be used to combine data from two layers with the same feature type (point, line, or polygon) through the web interface. However, the output is a new hosted feature layer with combined features and fields, not a hosted feature layer with multiple sub-layers.

To consolidate the layers as sub-layers within the hosted feature layer, follow the steps below for the ArcGIS Pro workflow. This workflow can also be used to incorporate layers with different feature types.

1. Open ArcGIS Pro and sign in to the ArcGIS Online organizational account. Refer to ArcGIS Pro: Sign in to access content from a portal for instructions.
2. Export each hosted feature layer to a file geodatabase. Refer to How To: Export a feature service to a file geodatabase feature class in ArcGIS Pro for instructions.

Note: 
If necessary, rearrange the feature layers' drawing order to ensure all layers are visible on maps. Refer to ArcGIS Pro: Set the feature drawing order for more information.

3. Share the layers as a new hosted feature layer to ArcGIS Online. Refer to ArcGIS Pro: Share a web feature layer using the default configuration for instructions.

Note 
Optionally, overwrite one of the exported hosted feature layers in ArcGIS Online. Refer to ArcGIS Pro: Overwrite a web feature layer for instructions.