Hello,
Great to hear that you have successfully extended the ArcGIS Maritime desktop solution to include your extra objects and attributes.
If by viewing in ArcGIS Pro, you mean the ENC Layer, there is no current capability to extend the portrayal. However, you can extend the the S57DataDictionary.xml and S52 lookup tables. By default, they are located at C:\Program Files\ArcGIS\Pro\Resources\ENC You can add your new objects and attributes to the S57DataDictionary.xml. This will allow them to be recognized when using the Explore tool. You can update the lookup tables to include your new objects and potential attribute combinations as well. By default the new objects will display as magenta "?" symbols if not found in those tables. Since you are unable to add new symbols, you will have to use an existing symbol in the lookup table to display the feature. If you require more detailed steps on how to do this, then please email the maritime team at maritime@esri.com and I can assist.
As for ArcGIS Maritime Server, you can extend and add new S-52 symbols. Similar to the ENC Layer in Pro, you have the S57DataDictionary.xml and S52 lookup tables. By default, they are located at C:\arcgisserver\directories\maritimeserver\maritimechartservice\controlfiles
As for S-52 symbols, you will need to create them yourself, we do not have an S-52 symbol editor, nor do I know where one exists. I do know other people have created symbols and plugged them into our solution. Some just took an existing symbol and modified the values. My recommendation is to do that and review the S-52 documentation on symbology for guidance. S-52 PresLib Ed 4.0.3 Part I_Clean.pdf and S-52 PresLib Ed 4.0.3 Part I Addendum_Clean.pdf. Refer to https://iho.int/en/standards-and-specifications if you don't have those documents.
Once you create the new symbols you just put them in the symshow folder, default location is C:\arcgisserver\directories\maritimeserver\maritimechartservice\controlfiles\S52PresentationLibrary\symshow and link them to your new entries in the S-52 lookup tables you modified. C:\arcgisserver\directories\maritimeserver\maritimechartservice\controlfiles\S52PresentationLibrary\lookup
If you are adding new S-57 Objects, those objects when entered into the S-52 lookup tables will need to be assigned a view group. If you create new view groups, then you will need to make sure those values are also added to the S52DisplayProperties.xml file at the bottom in the ViewingGroups sections. Then verify they are in an existing range under the ParameterGroup name = ViewOverGroups. This defines the layers that are created when you start the Maritime Chart Service's map service. If they are not part of one of the layers, then those features cannot be turned off in the display. This is all part of S-52 and defined in the documents I mentioned.
In order to see the new symbols, you must delete and rebuild your senc files.
If you require more detailed steps on how to do this, then please email the maritime team at maritime@esri.com and I can assist.
Thanks