The same way everyone else does...Not really.
If for example I wanted to be a professional aikido fighting athlete. How would I go about that?
Get into professional fighting athlete shape. Identify the sport you want to compete in. Learn the rules. Practice with folks who compete in that sport. Karate guys, Judo guys, BJJ guys, wrestlers, kickboxers... they all do it the same way.
Why would it be any harder for an Aikido guy to get resistance sparring from an MMA guy than a karate guy or a kickboxer or a wrestler? Aikido techniques are just as easy to ramp up the resistance on as a guard pass.