How to customize object type drop down list in advance search using presets in webtop 6.5?

Personally i dont think you can achieve this using presets.  Presets work on most of the situation but not on search.

You can easily achieve your requirement by customizing your advance search component .

The document below shows how to do it.

http://developer.emc.com/developer/downloads/Tips_CustomizingSearchInWebtop.pdf