A longer aerial can be fitted to a 2.4 GHz receiver, how easy it can be done will depend on the way the aerial is fitted to the receivers circuit board. Some receivers have the aerial mounted using a plug onto the board, in this case it is just simply removing the original and replacing with a longer version. many have it soldered to the board, this can be replaced by carefully unsoldering the old one and soldering the replacement on.A 2.4 GHz aerial is basically a length of coax cable with the screen removed from the end to uncover the inner, the length of the sheilded section is unimportant, only the exposed end need to be the correct length. These cables can be bought from specialist suppliers such as RadioSpares , unfortunately I do not have a link but have seen posts on older forums where modellers have done this. Some radio manufacturers may offer replacement aerials as spare parts and you might find longer ones available.As to why would a longer aerial be needed? Some receivers come with very short aerials which could be under the waterline if the receiver is mounted inside the hull, this will have a large effect on reception as 2.4GHz doesn't pass through water. An extended aerial can be led so that the active section is above the water, in an extreme case a submarine could be able to operate submerged if a longer aerial is mounted to the periscope such that it the end is always above the water at normal operating depths. (The failsafe could be set to surface if the radio loses signal.)
Jim