The Printer Control Panel software that ships with Apple Windows-marketed LaserWriters is simply emulation switching software, so it has no real role in producing the German letters. You can print the letters in different ways depending on which mode you choose for the printer (PostScript or PCL). For this article we'll assume that you're using PCL.
Given this assumption, you can change the symbol set used in PCL mode to one that contains umlaut characters. The ROMAN-8 symbol set, for example, contains the characters you're looking for. We've provided the PCL escape code sequence at the end of this article to perform this.
Of course, you first need to generate the umlaut characters on the screen. The character codes for "è, f, Ü, º" in the ROMAN-8 symbol set are 204, 207, 206, and 222 respectively. To generate these on the screen requires some combination of modifier keys (Shift, Option, and so on) to get the characters.
Here's the HP PCL escape code to change the symbol set to ROMAN-8. You should place this at the beginning of your text document:
<esc>(8U
The Printer Control Panel software determines that this is HP PCL and automatically switches the Apple printer into PCL mode.
To enter <esc>, press the Ctrl-P keys followed by the Esc key.
Note: You must use an ASCII text editor like DOS EDIT. You cannot enter the <esc> character in some Windows-based editors like Notepad.