The new SQL dataset beta feature could help you resolve this. It’s described here:
https://www.gooddata.com/developers/cloud-native/doc/cloud/model-data/create-logical-data-model/create-sql-datasets/
In Cloud, it’s already available. In Cloud Native it’ll be introduced once version 2.4 is released. You’ll be able to create the virtual column in the LDM modeler.
It still needs to be done in SQL, but you do not need to alter your database, but can use solely GoodData. Just convert the dataset to SQL dataset, add the custom column with some SQL expression, map it to new label, save the model and DONE.
If you are using workspace data filters, you need to make sure they also have the proper column for them in the SQL. It’s still a beta feature, so please be aware of that.