I dont know the SwissAddonFramework, but this works out on Standard:
Add an Item:
SAPbobsCOM.Items item = (SAPbobsCOM.Items)oCompany.GetBusinessObject(BoObjectTypes.oItems);
item.ItemCode = "Test";
int ret = item.Add();
Update an Item:
SAPbobsCOM.Items item = (SAPbobsCOM.Items)oCompany.GetBusinessObject(BoObjectTypes.oItems);
item.GetByKey("Test");
int ret = item.Update();