Hi,
Not sure why you are looking to have stock in transit. Required more details.
If it is for financial planning of Custom Duty payable of upcoming shipments then you may follow below step.
1. Create PO with confirmation control key as Inbound Delivery.
2. Based of vendor shipping document, create Inbound delivery( VL31N) with reference to PO.
3. Now, you can have report of all the incoming shipments /Inbound delivery in VL06.
4. Custom Duty Invoice Verification (MIRO) for Custom Vendor
5. Goods Receipt with reference to PO/Inbound Delivery.
6. Invoice verification for Main Vendor.