Lookup to the Activity entity (ActivityPointer)

Hi all,

I'm working for a customer that uses custom activity entities to registrate their done service activities. They now have a new request to connect two activity records so they know what activity is the follow-up of it's previous activity. I wanted to make a Lookup to the Activitiy entity (ActivityPointer), but that seems simply unable to create from standard CRM.

Anyone any idea if this is possible some way?

Thanks,

Marc