Is there a document that describes migrating a simple Java project to
support JSF?
Not currently AFAIK. If you find one or want to write/blog one we would
be happy to link for others.
The main steps, as you observe, involve adding the JSF facet (this also
requires adding the Dynamic Web Project and Java facets) and adding the
runtime jars to the classpath (which can be done from the facet install
dialog). To add facets to an existing project, right-click on the
project, select Properties and go to Project Facets in the dialog that
opens.