If you have tendencies toward high blood pressure, are pregnant, or have epilepsy or seizures, do not use rosemary oil. Otherwise, yes, one or two drops in your shampoo is fine. I use Suave's tropical coconut conditioner and it already has a little bit of rosemary essential oil in it, so I don't use any more than what's in there. Rosemary oil is mostly known for hair growth and for controlling dandruff a bit.
Don't put too much rosemary oil in your products. I made the mistake of taking a shower at night and using a drop or something in my no-poo solution for that one shampoo (I can usually use a drop of something, anything at all, and it doesn't bug me) and I had THE hardest time sleeping because my heart was racing from the rosemary oil. It was terrible. That pretty much convinced me that even if something is natural, it can be pretty potent and you can't just mix stuff together without knowing anything about it.
If you want a good degreaser, try a citrus essential oil, like orange, lemon, bergamot, lime, or the like.