You can't scrape off cultures from an already made cheese. It's a good question, but it will only create problems.
Here's a link on how to make a mesophilic MOTHER culture:
Cultures can only be created in labs, but that doesn't stop you from being able to make a MOTHER culture. The shelf life of a mother culture is only about a week. Three months if you freeze them in ice cube trays. But if you keep the culture in it's dehydrated powdered form, it can stay frozen up to 1 year AFTER the expiration date.
Freshness is important. So if you buy buttermilk, use it directly. Make sure you have the right amount and it's at room temp. Always remember that your buttermilk needs to be as fresh as possible.
Last point on buttermilk and yogurt as a starter: Buttermilk contains mesophilic cultures that are needed in hard cheeses like cheddar, colby, monterey. Yogurt contains thermophilic cultures such that are needed in cheeses such as mozz and Parm.