Yes you can but it is recommended that you remove the old color. It will also depend on the darkness of the color you had underneath. If you color over it with a lighter color you may have some of the old color mixing a bit with the new shade.
You can have that done at a hairdresser, but I would not recommend doing it on your own. I always heard that the colors can react badly and cause your highlights to turn a different color. A hairdresser can definitely do this because I had blond highlights and then died my hair brown with red highlights and it worked.
It is not reccommended that you dye over highlights, especially if your hair is another colour and you want to dye the whole haid. Your highlighted pieces may come out a different colour than what is on the box, but the only way to know for sure is to try.
Yes you can especially if it has been 4-6 weeks since your highlight treatment. Try using a temporary color which has less of the damaging chemicals to make sure you are getting the color you want. Make sure to condition your hair after you've dyed it and give it a deep conditioning treatment at least once a week.
Please don't dye your hair. let it be black or any color you choose. It consists of chemicals which may lead you to loose all your hair. Its better to have hair in any color than being without hair.