How can I enable manual payment methods for B2B customers?

At this time, it's currently not possible to set up direct bank transfers as a payment method within the Shopify Checkout for B2B customers. 

Shopify has received some feedback around requests for this functionality in the past, and this is a feature request that was pass along internally to Shopify Developers. Though you aren't able to set up direct bank transfers payments within your checkout, or direct debits to B2B customer accounts via your checkout, you can work with these B2B customers outside of the Shopify platform if you would like to accommodate this sort of payment process for them.