If you go to 'Patch' menu you will find a button called: 'Fixture Builder' If you press this button you get a new screen, press 'Create New' and you can start building your own fixtures.
If having difficult fixtures, I suggest you mail High End or post your requests in the 'Wholehog 3 Library Requests/Corrections' section.
As Pim said the fixture builder is the tool for creating libraries. There is a chapter in the manual that describes the process. However if you need a specific fixture it is best to check our download web page at: www.flyingpig.com/hog3libgen/
and if you can not find your fixture, then you should email support@flyingpig.com or post directly in the "fixture request forum" on this site. We can usually create libraries within a few days (media servers can take a bit longer).
The library is not on the download page, already checked before posting. The library I need is for a media server and was already requested. Will need to make some libraries really short-dated in the future. Thought there is maybe a software solution for that ...