Is there an attribute that merges StockItem with CrossItem?

Why not simply use [CrossItem] ? The POReceiptLineInventory attribute simply inherits from CrossItem and adds a filter to prevent you from selecting items that have status set to No Purchases. CrossItemAttribute itself inherits from the same base class as StockItemsAttribute  - you don’t need to use StockItem as well.

    [PXDBInt()]
	[PXUIField(DisplayName = "Inventory ID", Visibility = PXUIVisibility.Visible)]
	public class POReceiptLineInventoryAttribute : CrossItemAttribute
	{
		public POReceiptLineInventoryAttribute(Type receiptType)
			: base(typeof(Search<InventoryItem.inventoryID, Where<Match<Current<AccessInfo.userName>>>>), typeof(InventoryItem.inventoryCD), typeof(InventoryItem.descr), INPrimaryAlternateType.VPN)
		{
			var condition = BqlTemplate.OfCondition<
				Where<Current2<BqlPlaceholder.A>, Equal<POReceiptType.transferreceipt>,
					Or<InventoryItem.itemStatus, NotEqual<InventoryItemStatus.noPurchases>>>>
				.Replace<BqlPlaceholder.A>(receiptType)
				.ToType();

			_Attributes.Add(new PXRestrictorAttribute(condition, IN.Messages.ItemCannotPurchase));
		}
	}