Not sure about the TaxID, but you need to insert a record into SupplierList to link an Item to a Supplier. Item.SupplierID only indicates the Default Supplier - it does not create the SupplierList link on it's own.
This is the bare minimum statement to add SupplierList records for the default suppliers listed in your Item Table. This is assuming that your SupplierID field in the Item Table has the correct Supplier IDs, which it may not...
As always, MAKE A BACKUP OF THE DATABASE BEFORE RUNNING THIS STATEMENT!
insert into supplierlist (ItemID, SupplierID, Cost, ReorderNumber, MasterPackQuantity, MinimumOrder, TaxRate) select i.ID, i.SupplierID, 0.00, '',0,0.0,0.0 from Item i left join supplierList sl on i.id = sl.ItemID and i.SupplierID = sl.SupplierID where sl.ID is null