By default, you can set only meta description to you Opencart's home page. With this extension you can set meta keywords for Opencart's home page, also.
Meta Keywords for home page can be set in System -> Settings -> Edit your store -> Store tab -> Meta Tag Keywords.
This extension is included in Opencart SEO PACK PRO. It contains 20+ extensions and you can save more than $100 by purchasing the full Opencart SEO PACK PRO: