I am not sure if this correct and would need to test but if you give it a try I would think something like this may work

@cmd = N'select ' + @col1 + 'from dbo.testtable'

The other way is which may work is

declare @SQLString AS VARCHAR(max)

set @SQLString = 'select ' + @col1 + 'from dbo.testtable'

@cmd = @SQLString

My home server is done when I get into my office I will give this a try