Fishing wire is your friend when it comes to re-filling these, or in fact anything very thin that you can use to break the seal.
We used to successfully re-fill the original plastic cartridges using this method,
place the rod through the fixed end of the cartridge so the cartridge is effectively sealed at that end. fill with oil to slightly higher than the depth the seal will insert to. Then place the end of a piece of medium course fishing wire into the cartridge with a decent length sticking out. Then push the seal back down the rod and into the end of the cartridge.
The idea is that the wire creates a tiny break in the seal between the inner edge of the cartridge and the seal insert. This allows the tiny bit of excess oil you put in to escape through this tiny gap. The end result is the cartridge is only filled with oil and has no air in it.
The more effectively you can re-fill the cartridge with just oil and no air, the longer it will stay filled in use. Any air that remains trapped in the cartridge will heat up as the oil and air is forced through the damping valve. The heated air tries to expand raising the pressure in the cartridge and forces oil out. Hence a badly re-filled cartridge will not last long before signs of leaking oil become apparent.
I guess a needle would also do the job, as you could grab it with pliers to pull out if the seal was a really tight fit.
This has always worked for me. I preferred to use a lighter oil and adjust the dampening to a heavier setting. 2.5 or 5 weight.