Introducing Ranma to the Stargate world is one of the hard parts, trying to make it so it's not too 'out there'.
An organisation in the Japanese goverment, the JSDF Japanese Special(or Supernatural) Defence Force or another organisation; have an interest in Ranma. They could have always known about certain martial artists and their abilities, just not in any great detail. Reasons why they haven't involved themselves in the martial artists lives before now, could be for some cultural reasons.
But the goverment decides to involve themselves because they now know about the Stargate. Japan wants to send the best they have to offer, (this would work well with SG-Atlantis).
For Ranma, instead of just involving him from the streets of Nerima to the SG program he could have been working for the JDSF for some time. Reasons for that; they needed his help on supernatural or other unusal threats that they couldn't handle themselves. They don't need him all the time, just when the threats are extreme and no one else could handle the job. Ranma would have been on a number of missions before hand, letting him learn some miltary and other related tatics and procedures.
As for why Ranma would help with something like that; to get away from Nerima for short periods, high pay, learning new skills, challenging things that others wouldn't stand a chance against?
Even though Ranma would be a good choice for them to send to Atlantis or work at the SGC another point in his favour could be; on a previous mission he rescued one of the Japanese scientists due to be sent to Atlantis; she gave him her support when suggestions for other Japenese personal was asked for. This could be used if Ranma's more intense abilities are needed to be kept on the down low. I.e. only the Scientist knows some of what Ranma's capable of because he used his abilities to save her. Not that Ranma is keeping his skills hidden just that he only uses what he needs to win in any given situation.
While working with the JDSF on other missions (don't go over board on the number of missions he could have done) he would have to have used some of his more powerful techniques and skills; so his 'superiors' might not know everything but they know enough. Also that he gets the 'job' done; another reason why they would let him on the Alantis mission even though they should only be sending scientists (Other countries would send security, military personal mostly the U.S.)
With the curse, either letting his 'future co-workers' know or finding a workable solution is an idea. Waterproof soap, or the JSDF coming up with something. Nothing permenant though, because having 'her' being caught out would be a good plot point. On the other side, maybe having the Atlantis or SGC leaders know about his curse would work better.
I think sending him on the Atlantis mission would work better, but having him and SGC could be good as well.
If you wanted to involve Ranma with the SGC before season 9, then having the Chinese goverment involved could be a good idea, since they've know about it since season 6. Maybe a joint project with the JSDF after the Chinese catch wind of his actions at Jusendo and his involvment in other 'battles'. They already know about the 'Jusendo tribes' and some of their abilities, but have already been refused by them. The tribes not needing or wanting any contact with the Chinese goverment beyond what is necessary.
With the Nerima crew, initially keeping his work with the JSDF secret could be done by them giving Ranma a cover. Or if you can thing of something better go with it.
That's pretty much all I thought of so far. I do have a few ideas possible plots for what could happen when Ranma in a regular in Atlantis or the SGC. If anyone wants to take this up, go for it. I would but I'd probably never get it finished in my life time. I just write too slowly. I am working on another idea where Ranma gets taken from earth and crash lands on the Sodan planet; that's a purely SG-1 fic. But anyway that's something else entirely, so that's it for now.
