OK I see what you mean. The following should address it.
Code:
Dim oRng As Range
Dim oOriginal As Range
Set oOriginal = ActiveDocument.Bookmarks("MybookmarkName").Range
Set oRng = ActiveDocument.Bookmarks("MybookmarkName").Range
oRng.Collapse 1
oRng.Text = "toto"
oOriginal.start = oRng.End
oOriginal.Bookmarks.Add "MybookmarkName"
or alternatively
Code:
Dim oRng As Range
Dim oOriginal As Range
Dim sText As String: sText = "toto"
Set oRng = ActiveDocument.Bookmarks("MybookmarkName").Range
oRng.InsertBefore sText
oRng.MoveStart Count:=Len(sText)
oRng.Bookmarks.Add "MybookmarkName"