You do not need the Activiti WARs to integrate Activiti with Alfresco. Activiti is already bundled as an internal engine within Alfresco. I thought you wanted to develop / deploy actual workflows using Activiti that are to be run within the Alfresco Repository server.
If you want to use Maven, there is virtually no way to work without being able to fetch all the dependencies from the internet at least once (you can work in an offline mode later). Technically, it is possible to unpack the WARs and use all the JARs from that to build your project, but that'll only work for the direct project dependencies and not any of the Maven build plugins included with the SDK. Also, this way, your dependencies are effectively unmanaged as you include everything the Alfresco WAR contains, even if you may not want to or should not use some of the transitive / technical JARs.
Regards
Axel