I would recommend using Joomla over WordPress, that said it doesn't answer your question.
You can't make one CMS run inside the other. They will have to be two separate systems, and you will need to create a template for Joomla to match WordPress.
The way I would do it is as such:
Have WP installed on your root, then create a sub folder - something like products or store in which to install Joomla 3.x.
So accessing your store would be by going to www.example.com/products
You will need to manually add links to each site and it will be harder to maintain. If you have PHP/MySQL experience on the Joomla template you could create a query to the WP database to get article information so you could have "latest news" or something in the side coloumn.
Two side notes:
1) Don't try and have users get confused. Either register them on Joomla, or WP. Trying to do both will make it messy (if you even what user registration that is)
2) If you do use Joomla, and use a decent backup software like Akeeba then also get it to backup your WP database.