Yes, in most cases they can. Transfers and sightseeing can sit on your website as products in their own right, so a customer is able to book an airport pickup or a day tour on its own without buying a flight or a room from you at the same time.
A portal will usually carry airport transfers, private cars, day tours and activities, each with its own search, its own availability and its own voucher. Your customer chooses a pickup point and a time for a transfer, or a date and the number of travellers for a tour, and pays the same way they would for anything else on your site. You can also show these at the end of a flight or hotel booking, so the option appears while the customer is already buying from you.
What you can actually sell depends on which suppliers are connected to your portal. Transfers and tours come from different suppliers to the ones that give you flight and hotel content, so each one has to be connected and switched on before anything shows in a search. Coverage is uneven as well. A supplier can be strong in one country and thin in another, and smaller towns usually have far less to choose from than the main tourist cities. If you have your own drivers or your own tours arranged locally, most systems let you load them yourself so they sit alongside the supplier ones, though whether that is included is worth confirming for your plan.
A good next step is to write down the destinations you sell most often and the kind of extras your customers usually ask for, then check the live coverage for those places before you advertise them on your site.