If your data is being hosted as a Feature Layer, you should be able to add a field and perform calculation on it.
If you go to your Feature Layer -> Data -> Context Menu on the top right of the data -> Add Field -> Click on the new fields header -> Field Calculator.
You should be able to populate the new field with the data you want with this method.
Here is a picture of what it looks like when you have the Context Menu open.
I hope this helps!
- Jacob Watson