If you go to the Inventory Maintenance menu (from the Accounting Menu) and choose Inventory Transfer you will be able to select a harvested crop and transfer it to a supply. In your case this supply will be a seed that can be loaded into a distributor. In other cases this supply can be Feed that can be allocated to livestock.
When you do this you will need to enter a Unit Cost. This should be the current market price of the crop. The price will be used to record a sale from the crop enterprises and the cost that will be used for the seed when planting the new year's crop.