Cannot add a SimpleContent column to a table containing element columns or nested relations.
This is a known issue. However, the solution is not well known :)
To be simple and streight forward, I tell you the solution which I tried and it worked,
1. Note down the column names, datatypes and relationships of the table.
2. Delete the table.
3. Add new table and add columns to it.
4. Redefine relationships.
Best of luck.