How to import time with date
I don't think that this is currently possible. The time part will we stripped in CRM_Utils_Date::convertToDefaultDate() called here: https://github.com/civicrm/civicrm-core/blob/master/CRM/Contact/Import/Parser/Contact.php#L1909 

You could have an extension read an imported text field with a date string and set the proper datetime value for the custom field via the hook hook_civicrm_custom in an extension. But adding the functionality to core would most likely make more sense.