Hi,
There is no direct support for this but you can try to change the customer information in the cart by CartContext.AddOrUpdateCustomerInfoAsync.
SecurityContextService has also a method ActAs. You can try to use that one when you are getting the prices, adding item to shopping cart and placing the order.
If you want to login as another user please see Login an account without password. I’m not sure but it has to work for Litium 8 version also.