I have a linked table that I want to create some queries for. I would like to add an extra column to the table which is the sum of two existing columns in order to be able to use this in the query. So far, I've come completely unstuck. Any ideas please?

Thanks.

Storing a value that can be derived (calculated) from other columns in the same row, is considered a no-no ;)

If that's what you're doing, then rather calculate in the query. Else, perhaps explain again, and show some (smallish) sample data demonstrating what you wish to do?

If you actually wish to add a column to a linked table, then you need to do that in the backend.