Can a fighter jet land on a modified airliner?
Technically yes. But I am unaware of any attempt to do this with airliners.

But there were several designs in the past which used a big airplane or a Zeppelin as a mother ship, which took fighters with it for air defense. The B-36 was involved in several such designs.



In all cases the docking was made from below, because this gave the fighter pilot the best field of view and reduced the risk of a tail strike.

While the fighter in the picture above is a Republic F-84, McDonnell even designed one specifically for the purpose of being carried around by a bomber, the XF-85 Goblin. It used a rather complicated trapeze for docking and was then pulled up into the fuselage, as the picture with a B-29 below shows.



Your design proposal with the simple platform on top of the fuselage would carry a high risk of a tail strike if anything goes wrong, so it is unlikely that this will ever be tried for real. But when you change the airliner to a AN-225, the idea looks not totally impossible. The Russians tried to dock a fighter with a mothership as early as the 1930s. The picture below shows a TB-3 docking with a I-Z fighter, the first pair of aircraft in history to dock in flight.



The main reason for docking was the much smaller range of fighters. Today, air refuelling takes care of this deficit, so it is unlikely that anyone will see a benefit strong enough to justify the expense of modifying two aircraft and the risk incurred in the docking process.