Yes, you can do tricks like this by trapping everything, using a null trap for the detail, then selecting one big field for the whole line, making a calculated field for the changes you need, hiding the original field and exporting the table as fixed width text.
You'll need to replicate the logic of the traps for your append/header rows in the calculated field to avoid changing detail date, e.g. if you trapped the Append using the letters ABC, then define the calculated field using Extract() or Substr() depending on the floating trap.
Further to Olly's comments, I'd add that it might be prudent to be sure to disclose your manipulations of original data to whoever is using your end result, as one of Monarch's best features is that is not intended to alter original data in any way, and thus is a very trusted product.
That said, a calculated Character field with a formula something like this will help you make the substitutions that you need:
In this example, if MyField contains "Test Data", then the calculated field will contain "Actual Data".