placing numbers in front of existing numbers

I have an exported CSV file that needs to be uploaded into another system for a 911 database. I have over 4000 entries and in one column, I have the extension numbers of my users which is a 4 digit number. I need to place my 3 digit prefix in front of these 4 digit numbers for the database to work correctly. The 3 digit prefix is the same number for all cells. The merge cell doesn't work. I need to do this on a weekly basis, so hopefully there is a simple formula or feature that would work.

Thanks for any input you may have.

the concatenate function does exactly that:

=CONCATENATE("911-";A1)

Replace 911 with you extension.