Yes, you absolutely can. Look on-line or in shops near you for an external DVD drive. Here's a link to Amazon with some listed: amazon.co.uk External Optical Drives. The slim portables generally do not need a separate power supply but may need to use 2 USB ports on the machine to get sufficient power.
I'd also ask, why do you think you need one? When I bought my first netbook around 3.5 years ago, I also purchased an external DVD drive for it. I used it a couple of times but haven't used it now in several years, it sits in the cupboard unused.
If you intend to use it to watch DVD movies then that's a good use but for pretty much any other use there are other ways of doing the same thing without the need for the external drive.