Can I ask applicants to include a covering letter?

Unfortunately not, and here is why...

Disappointingly most job boards have not created the functionality to upload a covering letter, so it is technically impossible.

Job boards took this decision because job seeker behaviour is use the same application for every job and so they don't want to upload covering letters.

Additionally some employers 'abused' covering letters, wrongly assuming the applicant was motivated when they could easily have been desperate for any job.