You have two options, use the existing hole axis and redefine it, or create another axis through the hole as the datum axis. I will try to explain both.

The first method involves selecting the original axis of the hole you want to use a datum and choosing Properties. You can change the name, type and placement. You will want to choose one of the two datum types depending on your standards, then you will want to choose the In Dim placement type and choose the hole diameter for the hole you wish to use as the datum.

The other method is similar but you Insert a Model Datum: Axis first. I think that if you do this in drawing mode, it Automatically changes your type, but I'm not sure. Just be sure in both methods that you set the type to a Datum type and choose your placement correctly.