If you have advanced logic you would like to implement I would hide the current implementation using an editor descriptor and then implement a new custom property which uses the categories as underlying data store. 

For most scenarios you can just override the editor descriptor. Check out 

http://geta.no/blogg/improved-categorylist-editor-descriptor-for-episerver/

For an example of this 

If you want to hide default implementation check out 

http://joelabrahamsson.com/hiding-episervers-standard-category-property/