to get Ranma into any mecha at all will involve getting him to give up on the idea of fighting whatever it is that needs fighting in hand-to-hand
Shouldn't be that difficult since Ranma has used other tactics besides hand to hand. Hand to hand isn't even usually his first option. He first tries to talk it out like with Ryoga or Ukyo. Failing that he tries to manipulate the people like how he did to Principal Kuno to get the coconut or constantly does to Ryoga. Then uses hand to hand but not always sometimes he'll try another tactic like drugs (tried to drug Kuno's food used knock out powder on Shampoo), magic (suggestion incense used twice, girl repellent), or offering to date them (like the little boy or Shampoo so as to manipulate her).
Getting Ranma to use a mecha would probably be very easy. He would most likely see a mecha as just another tool to use the same way he did the other tools he used, like the magic fire breathing sentient staff, various implements he grabs up to use in combat, or when he tried to get the magical dogi. Unlike fanon Ranma has no problem with using weapons, using power-ups (tried to get two of them), or getting help when he thinks he could use it or needs it (he has had people like Akane help him - example had Akane sew magic girl repellent onto Happosai for him). If his abilities are insufficient on there own, he would use a mecha.