You can do it via schema manager screen. Following are the steps for reference :
1 – Login to Lumenore Homepage and navigate to Datasets tab(database icon)
2 – Select the schema and click on manage schema (if already created) or create schema through a dataset.
3 – Once in schema manager screen , switch to the variables tab
4 – Click on ‘Add Variables’ button, it will open a popup window
5 – Select your preferred language and write the expression. Validate it to check for any potential errors and save it.

Variables created through schema manager are global by default however user has an option to govern visibility of variables in other modules through availability option present in variables tab of schema manager.