If you want to you can add the freight condition in PO itself.
Just create a new condition type for freight or used the same which you have in MM pricing procedure, maintain as subtotal 6 against that freight condition.
Goto your tax procedure and maintain 362 against the condition BASB.
Now create a new PO and add the freight condition and check the in MIRO.
If your client wants to add the freight condition directly in MIRO then nothing can be done , just tell your client this procedure and do let me know.