Servus bultaco290,
As I recall KTM/WP says 15mm is max. preload on the 43mm USD forks. This is achieved with spacers on top of the springs. If this spacer is too big, you can a) damage your forks and b) with too much preload your suspension will not work alright.
So if you really need more than 15mm to get a static sag within spec your fork springs are probaly sagged out . But be careful, the thickness of the spacer is not equal preload!!! Stock sag on these bikes is 6mm with the spacer being much thicker.
I'm running 12mm of preload on my 01 520EXC and get 30-35mm of static sag!
Michael