
2 answers
Meaning when the event is related to a contact?
Events and Tasks are picky because the Name and Related To fields are actually combinations
Name can be Lead OR Contact
Related To can be Account, Opportunity, Asset, etc.
So they can't have lookup fields because technically the system doesn't know which object you are looking up to.
Now, once Custom Lookups on Activities come out, game on
http://releasenotes.docs.salesforce.com/en-us/api_cti/release-notes/rn_sales_activities_activity_custom_lookups.htm