If you google a bit on this topic, you'll find several blogs and articles on the subject of Forms to Apex conversion. The basic conclusion is: don't bother using the converter. Just build a new application from scratch. The differences between Forms and Apex are too big.
Here is one presentation:
Oracle Forms and Application Express are significantly differently products that implement the concept of database application development in significantly different ways. Because of this, achieving a 100% (or even close) conversion of look and feel and functionality is beyond the scope of the conversion facility. I think the best you can hope for is a utility that can speed up the initial stages of the conversion process.
Thank you for your answers!