Yes it can be done, you'll need a syringe and a lot of time. You would be better off taking them apart, clean the nasty stuff out of them, yes they do get dirty inside, then refill them before you seal them back up. It's not that hard of a job. I believe there is a how-to somewhere on the forum, there must be, I did mine.
Mantra, I've done it both ways, through the airline hole and from the bottom when replacing the seal. Trust me, just pull them, clean them and fill them then, It's a lot easier and less time consuming.