If you are planning to list this under your work history, that means you are claiming points for it and will need all the documents including the letter. If you are listing this under your personal history, no matter how many additional documents you provide they will not count this experience for you. So you might as well save the hassle for yourself, or just upload it under work history and claim points for it. If you upload it under work history and do not attach all the appropriate documents, you will be rejected, despite having more than 3 years of experience already claimed.