Just create a action rule off the drop down field, in the "run these actions" dropdown select "Set a fields value". From the new window in the "Field" box select the new field you want to populate the text too. In the "value" field click the Fx button, insert a field of group button, then select the dropdown field where your manual choices come from.
Now every time they change that drop down field whatever value they put in is also transferred to the other field you wanted populated. Repeat as many times as needed. Hope that helps.