Yes, of course you can. If you wish to arrange your own volunteering event, it should align with the Mitie Foundation's scope of supporting education, employability or enterprise related activities.
If you wish to undertake a more practical event, there are many charities and community organisations that would welcome people with appropriate skills and resources to undertake maintenance, gardening, cleaning etc. However, it is important that you ensure that the work is within your skill set and that it will not expose you to undue risk of injury to yourself or harm Mitie's reputation. Appropriate clothing and personal protective equipment (PPE) must also be worn at all times you are on-site, and a risk assessment by a competent person is undertaken where necessary.