The most likely way to make this happen is by using the intern selection process, as well as the internships themselves, to make students at the college or universities aware of the firm and the sorts of jobs that might be available.
When the firm selects interns for its program, it will surely interview more applicants than it actually selects. The people who interview will then be more aware of the firm. Of course, the people actually selected will be more aware of the firm as well. It is also likely that more of their friends will come to be aware of the firm. In these ways, the firm will be putting itself on the students' "radar." This will be likely to help the firm generate future applicants for jobs.