When you create a vendor bill, Core prompts you to link existing, approved time and expenses to the vendor bill. But if you have created the vendor bill and want to link additional time entries to it (maybe because you didn't choose to link them before or because you created the entries after you created the bill), you cannot do so from the Time Entries screen. You need to enter them as new line items in the vendor bill and save it, provided there are no bill payments associated with it. Core automatically creates new time entries for the line items, so you need to delete the original ones in the Time Entries screen to prevent duplicate entries. Check the Core Help for details on editing vendor bills.