I am still not sure I follow you.
In OWB you can insert a WBS item (task or phase) in the middle.
If that is a task, then that will just be in that position in the sequence and will not change the existing tasks.
If that is a phase all existing tasks below that until the next phase will be under the new phase.
That is the standard OWB functionality.
If you do not want any of the existing tasks to be under the new phase, then you should create the new phase at the bottom of the list.
Maybe that answers your question.