If your form does not have code, you could try qRules. It contains a command called Insert. This command allows you to insert a new row in your repeating table. Then, you can set that newly-inserted final row equal to the row that the user has selected in the 2DS (secondary data source). The process is documented here.
If you are already using code in your form and therefore cannot use qRules, well, the solution is to write your own code to insert a row into the target repeating table and set the row equal to the source in the 2DS.
Good luck!