Yes, end mills can be resharpened, however you need a special sharpening grinder in order to do this properly. Sharpening bits makes more sense with larger diameter end mills that are very expensive. If you are paying to sharpen your bits then it does not make good financial sense to sharpen smaller bits (1/8″, 1/4″ or smaller). The smaller the diameter of the bit, the more difficult it will be to sharpen it as well. Many bits that you will use on a small CNC router are going to cost less than $40 and good quality carbide bits will last you a very long time, especially considering the workloads derived from smaller CNC systems.
As an example, if I have a 1/4″ end mill that I have used for a year and paid $29 for it, I would simply throw it away and buy another before I would spend $20 to have it sharpened.