Converting Data Type of a full Column/Field
I'm not sure what you mean by 'whole field", but this is how you'd convert an Integer to a varchar string.  The 'second' select proves it's a string because you can concatenate brackets around it.

You'd apply the same technique when selecting rows from a table.

DECLARE @IntColumn int = 5
select convert(varchar(20),@IntColumn)
select '[' + convert(varchar(20),@IntColumn) + ']'