java - add an element to XML documnet

I know how to remove the element using xpath like below, but dont know how to add an new element. Please advise

For example, I use the following to remove the element using xpath
elem = getElements("./student:enrolment", std);
elem.detach();

Now I want to add the enrolled course to the element <./student:enrolment", std);
i want to add the following element

<std1:enrollendCourse>
      <std1:course>Law</std1:course>
</std1:enrollendCourse>



please advise