Can I use a merge field with a funtion like GETFIELD("KON"+$mes) so it act as another field ? if not can I use another way to acomplish that ?

Sure, you can do that. Keep in mind that script variables with a single $ are only available while the script is running. If you want the variable to persist, use a variable defined with two $'s, as in '$$mes'. Also, while this works fine for extracting data from a dynamic field name, it may not show the dynamic field contents unless you add a Refresh Window[] step after setting the variable in the script.

You could also use a global for the field name, which doesn't have the refresh issue:

getfield("KON" & gFieldName )