Will the import function allow me to start my development from just-another-Wordpress-theme, or is this for transferring themes between ThemeFrame installations?

Themeframe is use to only create Themeframe themes. If after creating a theme you export the settings you can reimport them at a later time to make changes to a theme.

Every theme created has different ways of handling various parts of the code so it would be near impossible to have a tool that could edit any theme.