There are a few ways to do this. The first would be the Action Builder (Tools - Action Builder). What you can do is create a new Action where when the drop-down list is changed to set the value of your text field to the value of the drop down list. Just create a new action, select your drop down list, then in the Result section select "set the value of a field" , select your text field, then from the drop down box select "to the value of the field" and then select your drop down box. As you change the drop down box the text in the field will match it. The second would be to set the binding of the 2 fields to the same. In the Object tab select the Binding tab and in the Data Binding (Open, Save, Submit) field put something like Field 1. Make sure to do this in both the text field and the drop down box Binding tab. Now both fields are bound to the same data. Change the drop down and it will change the text field.
**Edit: I guess I should read a bit more. Are you saying that you have 30 drop down boxes where all 30 of them possibly need to be in 1 textfield?