For my Pulsar 220, I started using Motul 5100 15W-50 engine oil. The last time I changed my engine oil, I used only 1 L, whereas the actual requirement prescribed is 1.2 L. It's been two weeks since I've changed the oil, is it ok for me to add 150 or 200 mL now? Is it ok to add new oil to already running oil?
It is absolutely okay to add the extra oil at any time. Add fresh to old oil whenever the vehicle (whatever type of vehicle) needs it. For best results, use the same brand & weight of oil you used in the first place. While not absolutely necessary, you'll avoid issues by doing so.
Ensure you double check the amount of oil which is in the motorcycle already and do not overfill it. If you overfill it, you run the risk of blowing out seals/gaskets, as well as risking internal damage if you put way too much in. A little over probably won't hurt anything, but really, just stay at the proper level and you should be golden.