Tableau will not ever change data in your original database, and for good reason.
However, if your desire is to allow other Tableau users and workbooks to use the same calculated fields without having to redefine them in each workbook, you can do this without changing anything in your database if you are using Tableau Server. Create a workbook with all the desired calculated fields, then publish the data source to Tableau Server by right-clicking on the Data Source, then select Publish to Server:
Then, in other workbooks, instead of connecting to the original data source, you and your other users, to whom you have granted permission, can connect to the Tableau Server Data Source, where they will have access to all the calculated fields you published. This works for live connections as well as extracts.
I hope that helps.