You can add custom clothing and grooming options either via the Fab marketplace (Fab.com) or via the Fab launcher within Unreal Editor or using third-party applications such as Metatailor. Note that due to the custom nature and rigging required for Metahumans to work correctly, this can be tricky, so ensure you conform to Metahuman guidelines and content is made for Metahumans in a variety of scales and sizes.