Yes you can. I do it all the time. A simple enema can be done in the shower. I use water equal to body tempature while in the shower. I use a douche nozzle while standing up, I use soap as a lube and insert it. Have the water from the shower run down from the chest to the stomach and beyond.
I squeeze the hot water bottle to give me a fast fill and then hold it for a few minutes. I expel while showering. I sometimes take 3 or Enemas while showering.
If you use this during the day time and are planning to walk around, suggest to wear an adult diaper as liquid will expel.