Can a player on the back row come up and hit a ball at the net?

Yes. A player on the back row come up and hit a ball at the net.

Technically you can, but, if you attack the ball above the plane of the net you must start your attack from behind the "attack line" or "10' line" or "3 meter line". If you contact the ball below the plane of the net you can even jump front row and attack

The rule about back row attack depends on a couple of factors: in your question, a back row player is at the net so they are already in front of the attack line and if they jump to attack the ball, the ball must be at least partially below the net at the point of contact. If the backrow player in front of the attack line completes an attack where the ball is completely above the level of the net, then it is an illegal backrow attack. This rule is the same at all levels of volleyball. There are times where a backrow is quite tall relative to the net height and it is possible for a tall backrow player to hit the ball completely above the level of the net without jumping to the opponent's side of the court when the player is in front of the attack line where again, it is an illegal backrow attack.

A back row attack in volleyball occurs when one of the three back row players attacks the ball and contacts it at the top of the net.

In a back row attack, the back row player jumps from behind the white line, also known as the ten-foot line, or the three-meter line and contacts the ball.

The back row attacker must jump from behind the three-meter line. If the player attacks from in front of the three-meter line, a penalty is called.

Otherwise the back row players can either bump or set the ball over the net if they are past the 10 foot line and are closer to the net than the 10 foot line.

Yes there's no rule against that

if they can get there in time.

PS..."Baseball"?