It is possible, but it requires materials and expertise. The hydraulic fluid leaked for a reason: either the closer body has a crack (in which case it would just leak out again), or more likely one of the hydraulic seals (o-rings) has given out. So to fix a leaky closer, you need to completely disassemble the closer, replace the offending part, and then replace the fluid. It is also essential to put in the right amount of fluid, no more, no less, so be sure to consult with the manufacturer.
Some companies do this work. Often companies that have "door closer" or "door check" in their name rebuild door closers.