This is my sample code:

Dim webpart_id As String = CType(PartInstance.GetValue("WebPartControlID"), String).ToString
If Not CMSContext.CurrentDocument.DocumentContent.EditableWebParts.ContainsKey(webpart_id) Then
CMSContext.CurrentDocument.DocumentContent.EditableWebParts.Add("new_webpart_id_key", "new_webpart_value")
End If

It does not seem to add (as in insert) a new key/value item to the DocumentContent database column. Instead it replaces the only existing key/value pair.

What is the best way to insert a new key/value pair to the DocumentContent database field?