Is there a way to use the page up/down keys in Voyager to scroll
up/down?

As a workaround it may be possible to do that:
I know that there is a MUI class capable of reading those keys, so is
there maybe a tool that can convert page up/down key presses to
Shift+Arrorw-Up/Down?

Try using the numeric keypad's PgUP and PgDn. They work as you would expect the seperate page up and page down keys to.

yes thats true in Voyager but APDF for example only supports "Shift+Up/Down".
As there is a MUI class which can handle them, it should be no problem for MorphOS to map those keys to "Shift+Up/Down" (or give us the possibility to map them to whatever we want), which would be great in Voyager or APDF!

anyone?

If you use an USB keyboard, then you could use Posiedon to map those keys