Forms :: Data Entry Items Only If Previous Has Value

I'm creating user form items (text boxes/combo boxes) for a user to enter data in. In this case it would be for a part. What I would like to happen is, if there is more than 1 part, then after the first part is entered, another part entry item is added below the first one but only if the first one has a value - for a virtually unlimited amount of parts. BUT if there is only say, one part to be entered, then the next section of selection appears directly below the blank next part. Sooo, something like this.......

Invoice:

Part1: (THE PART HERE)
Part2: (THE PART HERE)
Part3: (THE PART HERE)
...
...
...
...
Partn: (THE PART HERE) - blank

(aButtonThatProcessesThis)

OR.........

Invoice:

Part1: (THE PART HERE)
Part2: (THE PART HERE) - blank

(aButtonThatProcessesThis)

Is this possible?