Text column

Hi everyone. Most likely silly question...

How to add a text column to an existing table with only numeric columns in it? Say I need a new text column which is called Store and only one cell with a text value XXL. 

I couldn't figure out how to do it so I've added a custom coumn (copy of an existing numeric column from that table) and replaced current numeric value with the text (XXL). Problem is that this value is updated which means the replace function doesn't work anymore. How do I then specify in the "Valye To Find" field that I need all possible values (to include any updates) that appear in this cell to be replaced with the text (XXL)?