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.

Yes there are multiple ways to do this within Lumenore.
In datamagnet ,you have an email component that can send email to any address configured within that. One can send emails at any instance while creating a pipeline. Regarding the threshold value mentioned above there are multiple ways :

1 – Use filter component in the pipeline and check for the field value through built in editor. If it exceeds the threshold simply apply an email component and send email

2 – One can utilize Simple Evaluation component for evaluation of given condition i.e. whether it is a field in your dataset that needs to be evaluated for threshold value breach or you can create a variable to keep track of the same, both can be evaluated in this step and based on result user can attach email component to get the job done.