With hemihypertrophy, of course you already know this, your right leg has grown at a faster rate than your left. because of the overgrowth, both the skeletal structure and muscular structure are larger than your left leg. bodybuilders experience this when they have a "lagging" body part. but all muscles function in this manner:
"The emerging theory behind skeletal muscle hypertrophy is that a bout of exercise causes protein degradation or damage (myotrauma), which leads to a period of enhanced protein synthesis or supercompensation when the bout ceases (Zatsiorsky, 1995). This increase in protein synthesis not only repairs the damage from the bout of exercise, but also makes the muscle stronger and therefore more resistant to future damage. The steps involved in muscle hypertrophy are:
Mechanical Stimuli --> Cell Damage --> Cell 'Clean Up' --> Cell Repair --> Cell Growth"
To put it simply, all muscles are the same, but you have more on your right leg.
So here's the game plan: Add muscle to your left leg to catch up with the right leg, BUT limit muscle growth to your right leg so the left leg can catch up.
I agree with iron fully. performing single sided exercises and working your right leg with lighter weights can help greatly. The only thing i want to add is that you need to perform exercises both legs at the same time so functional strength of your two legs dont match each other. Performing squats at a weight where your left leg can cope, enhances and builds muscle on your left leg yet it is at a weight low enough where your right leg isnt worked enough to grow in size. Performing squats will equally strengthen both legs and allow your left leg to catch up to your right leg without cause your right leg to further.
Think about it this way. say you have 2 runners running a relay race. you have a faster runner in front of a slower runner, but the slower runner has to give the faster runner the baton so the faster runner can run. if the faster runner runs too fast, the faster runner wont be able to get the baton. so what does the fast runner do? it slows down to the same speed as the slower runner so it can grab the baton. your right leg is the fast runner. if both runners are running at the same speed (is both legs are being being trained with the same weight on a squat exercies) then they will be at the same level and they will progress together. (sorry for the dumb analogy. thats how i learn best)
Perform bilateral exercises that put both legs at the same level. Perform exercises that emphasize and train your smaller leg so it can catch up. Also, if you're able to, read up on lagging body part in the bodybuilding frame of reference to better understand what exercises can benefit you in your goals.
In regards to steroid usage, i'm not sure what your state of health is. I know that hemihypertrophy greatly increases chances of liver and kidney adverse reactions and chances for cancer and tumor development. They use of steroids may place great strain on your internal organs that may harm you more than help. Also, steroids do not discriminate where they effect. Any muscle trained will be aided by steroids to increase in size and strength, even your right leg which is something you do not want. the use gear that causes localized growth would actually work better to better aid you in your goal of increasing the size of your left leg. You may want to look into and read up on igf's ability to promote localized growth.
BUT please exercise and have both legs at a state where they are remotely on the same level and have your diet in check before trying anything. I hope this helps you in some way. My apologies for the length.