Yes, you can add 200mg EW of deca and this may help. I would also add 3g of fish oil per day. If you are taking 3g of fish oil already, I would up it to 5-6g. If you aren't wearing a knee sleve, I would start doing that as well. It will help to keep your knee warm while you train and play sports. Pay attention to what irritates your knee the most. Certain exercises may put too much pressure on it. Certain machines may be worse than others. We have 5 different leg press machines at my gym. Some of them work fine for my body, while others will cause knee pain for me, so I avoid the machines that don't work for me. Leg extensions are another one that can cause problems. When exercising, make sure that you are using the muscle to lift the weight and not the joint. Lower the weight and fosuc on contracting the muscle on each rep, instead of just using brute force to drive the weight up. 

Also, be sure to ice after you are training, 20min on. keep the inflammation under control. If it gets consistently painful, use a NSAID like motrin 800...not for the pain but for the inflammation. It would probably also beneficial to do some rehab work on that knee. Deca may mask the pain, but it is not going to fix the problem. If you don't deal with the actual issue, you could make the knee a lot worse, pounding on it, while your body is unable to send pain signals to your brain. Remember, pain is your body's way of letting you know that something is wrong. Dont just mask and ignore it