Use IntPtr constructor:

Dim SomeInteger As Integer = 2000000000  'Not Long!!!
Dim FormPointer As IntPtr = New IntPtr(SomeInteger)

Caution: in the .NET framework the old Long data type is now Integer (32-bit). In .NET framework Long data type is 64-bit, i.e. in all API function declarations you should use Integer instead of Long. For example:

Public Declare Function GetWindowLong Lib "user32" Alias "GetWindowLongA" ( _
ByVal hwnd As Integer, ByVal nIndex As Integer) As Integer