I haven't used Visio myself to do this, but the user guide says you can use whatever tool you prefer to create XML files containing BPMN 2.0 - so assuming you can do that with Visio, I think it should be possible.
You deploy processes into Activiti either programatically via the API, or if using Activiti Explorer you can use the option Manage > Deployments > Upload New to load a process or processes. This can be a single XML file, which must be suffixed .bpmn20.xml, or a .bar (business archive) file which can contain multiple resourcess. This is all covered in more detail in Activiti User Guide.
