Yes, you can. A back office system is meant to hold every booking you take, not only the ones that come through your website, so your team can create a booking by hand for a trip sold over the phone, at your counter or arranged directly with a supplier. They enter the customer, the travel dates, the supplier, what you paid and what you are charging, and from that point the booking sits in the same list and the same reports as everything else.
This matters more than it looks. Your sales figures, your profit on each customer and your list of who still owes you money are only as complete as what is recorded. If a large part of your business is sold offline and lives in a notebook or a spreadsheet, every report you pull will quietly understate your real trading, and your accounts team will keep chasing the same information twice.
Be clear about one limit. A booking you type in yourself is not connected to the supplier, so nothing will update on its own. If the airline changes the timing, or the hotel cancels, or a ticket number is issued, someone on your team has to go in and record it. Bookings that came through a connected supplier can usually refresh themselves, and manual ones cannot. How much detail you can capture on a manual booking depends on how your system is set up.
If you are considering this, ask to see the manual booking screen before you commit, and check that it captures your cost, your selling price and the supplier reference, because those three are what your reports and your reconciliation will rely on later.