No, this is not possible. It can edit a PLD file though, so you could convert the PLL file to a PLD file, edit it with PL/SQL Developer, and convert it back to PLL.
You can also set your external editor in Oracle Forms to PL/SQL Developer.
PLD is the extension of a PL/SQL Client Library file in text format. You can convert between PLL (binary) and PLD (text) from the File menu in Oracle Forms.