Hi Mohamed,
When importing the lines, I would try to have NO inventtransIds in the staging table. During staging to target, the insert method of the quotation line should be triggered which would then create the InventTransOrigin record and should then also generate the InventTransID. I'm not sure if the quotation line was already supported out of the box. Otherwise, try to have a look how it is done at the standard entity for sales order lines.