One approach is to host a contest for your students yourself. You can either 'force' participation by including it in the lesson program or make it extra-curricular, where you can give additional incentive by handing out 'bonus points' to grades for all participating students. For the students that are good enough and enjoy your contest, the contest can strongly encourage them to participate in the olympiad.
An advantage of this approach is that you not only encourage the students you think are 'suitable' for the olympiad, but simultaneously have an activity that can motivate the other students into programming and algorithmic thinking and, ultimately, your course.
The main disadvantage is that this will take time and some skill to pull it off successfully. This may be less daunting than it seems: you can get questions from various sources and relatively small contests need no complicated submission software. However, I'll refrain from discussing further how to host such a competition; that would be worth a question on its own.
Then again, I agree with @Itamar Green that you likely need someone to deliberately invest time to encourage the students, so it is likely the time has to be spent anyway.