Update an object's unpopulated members with values from another using:

Mapper.Map(customerViewModel).OnTo(customer);

When merging collections, objects are matched by id (configurable if necessary). For example:

var source = new Collection<CustomerViewModel>
{
    new CustomerViewModel { Id = 1,    Name = "Rod" },
    new CustomerViewModel { Id = 2,    Name = "Jane" },
    new CustomerViewModel { Id = null, Name = "Freddy" }
};
var target = List<Customer>
{
    new Customer { CustomerId = 2, Name = null },
    new Customer { CustomerId = 1, Name = "Bungle" },
    new Customer { CustomerId = 3, Name = "Zippy" }
};
var result = Mapper.Map(source).OnTo(target);

In this case:

• source[0] is matched to target[1], but target[1].Name is already populated, so it isn't changed
• source[1] is matched to target[0], so target[0].Name is changed from null to 'Jane'
• source[2] has no match in the target collection, so a new Customer is added with CustomerId set to null and Name set to 'Freddy'
• target[2] (CustomerId = 3) has no match in the source collection, so it isn't changed
• The original List<Customer> instance is maintained and assigned to the result variable