If you wish to do it with VBA, just add a vbCrLf to your line of code. This
example puts in 6 lines automatically after the bookmark text is added:
Application.ActiveDocument.Bookmarks("bkcombo3").Range.Text =
IIf(IsNull([dx3]) Or Len([dx3]) = 0, ".", [dx3]) & vbCrLf & vbCrLf & vbCrLf &
vbCrLf & vbCrLf & vbCrLf