You certainly can create app packages without Visual Studio. After all this packages are simple cab files (how to create cab file).
But I would suggest you to use Visual Studio to create at least one package to get correct package structure. After you do that you can easily generate your apps using Powershell for example. One thing to remember - each app should have unique ProductId so you need to generate it for each app.
EDIT: Thanks to Martin Bäckström that he pointed it out - app packages are not cab files they are zip files. They are actually packaged the same way as Excel and Word docs.