Hasan,
Myself, or any of a number of Shopify Experts, can help with this.
Minimal is a responsive theme, so it's meant to adapt to various screen sizes. The white space where you indicate you would like a sidebar will not always be available, especially on smaller screens.
Instead, it would be possible to insert a sidebar in the space currently occupied by the fourth row of products.
I would be happy to help with this. Please send me a line, email me ( tim at mrgossett dot com ), or post your email address here if you're interested.
Regards,
--
Tim
