How null values was entered to datetime prior to Nullable types?

Nullable types was introduced in .NET 3.X

No, they were introduced in .NET 2.0. See the documentation.

How do i created a property of type datetime and make it accept null so i can send null values to my DB table column?

You couldn't. Typically people used a magic value of DateTime.MinValue to represent null, and then made sure that their object/database translation layer converted that to a SQL null appropriately. An alternative was to have a separate bool property to indicate the presence of a "real" value - or a separate type like SqlDateTime.