Sometimes employers will request additional documents but not properly configure their job posting to accept the additional documents. In such cases you will need to combine the documents to your resume before you upload it.
If you have already applied through the system, then you will need to withdraw your application in order to apply with the more complete documentation. What happens when you withdraw is different depending on if it is On Campus Interviews or a regular Job postings. With On Campus Interviews, you may withdraw and reapply an time during the application period. The employer will only see your last submission. With regular job postings, the employer will often opt to receive emailed copies of all resumes as they come in. In such cases, you still have to withdraw in order to get the interface to re-apply, but withdrawing does to recall any previously submitted documents.