No, it is not possible to directly import geodatabase feature classes to a hosted feature layer as a sublayer in ArcGIS Online. However, it is possible to export data from the hosted feature layer to import the feature classes using the Feature Class To Geodatabase tool in ArcGIS Pro.
- Log in to ArcGIS Online and export the data from the hosted feature layer to a file geodatabase. Refer to ArcGIS Online: Export data from hosted feature layers for instructions and more information.
- On the item details page of the exported file geodatabase, click Download to download the .zip file to the computer, and extract the file.
- In ArcGIS Pro, add the feature classes to be imported to the hosted feature layer to the map.
- Import the feature classes into the file geodatabase downloaded in Step 2 using the Feature Class To Geodatabase tool.
- On the View tab, in the Windows group, click the Geoprocessing button.
- In the Geoprocessing pane, search for and click Feature Class To Geodatabase (Conversion Tools).
- In the Feature Class To Geodatabase pane, for Input Features, select the feature classes from the drop-down list.
- For Output Geodatabase, select the file geodatabase exported from the hosted feature layer by clicking the Browse button.
- Click Run.
- In ArcGIS Online, add the output geodatabase and publish its contents as a hosted feature layer. Refer to ArcGIS Online: Publish the contents of a file geodatabase for instructions and more information.