There are ways to do it but as a purest, I like to remove, check and replace things.
You haven't said when the fork's internals were checked last (not a criticism) so hard to say what other issues the bike may have.
@Igofar - Larry, what's your opinion on the subject?
Sure, throw me under the bus
I'm not of the camp of using seal mates or 35mm film, to scrape/clean seals, as it does more damage than good.
When seals are installed on the tubes, you should use a condom or plastic film over the edge of the tube to protect the soft sealing edge of the seal from damage....
Yet, folks want to shove a hard piece of plastic down there and twist and scrape things etc.
Since oil has been lost, the OP has no idea what damage/wear has been done to the bushings inside the tubes, or how much fluid has been lost etc.
Adding some, could unbalance the forks and cause handling issues.
The forks need to be removed, taken down, cleaned, and rebuilt correctly, if you want to do the job correctly.
Yes, you could dump oil in there, but if your opening things up, why not do it right, instead of doing it twice?
To add fluid/oil you need the tubes straight up and down, collapsed, and the oil needs to be measured from the top of the tube, to the surface of the oil etc.
If you use ONE measuring cup, and dump X amount of oil in one leg, then refill the cup, and do the other, you'll have different levels in each fork, as the fluid clings to the sides of the measuring cup.
You can get by that by using TWO measuring cups, at least that way, they will be even, and twenty minutes after you finish the job, you'll find about 1/2 inch of oil in each cup
I personally would rather fix things correctly, while the bike is safe in the garage, rather than take short cuts, and can go south somewhere on the road.
A leaking fork seal can/may also take out your brake pads, and compromise the ability to use your brakes when needed.
Back under the bus I go....