We have implemented a powerfull opt-in import features which will use the opt-in or double opt-in mechanism while importing contacts. 

You need to make sure your default headers are set, please check how to do that here.

You also need to make sure your preferences for Opt-in are set, check how to do that here.

First you'll have to activate this setting in your account settings.

Click the My account menu from the left navigation menu and click the settings submenu.

Once clicked, you'll arrive on the settings page, scroll down until you find the following section.

Click the account settings link and inside the popup search Import - Show optin questions.

Check the checkbox to activate this feature and click save.

Once activated, you have the following options for your fields.

Click the segment section in the left navigation menu, and click the contacts submenu.

To crosscheck this feature, launch an import of contacts by clicking the button inside the contacts page actions menu ( on the upper right side of the screen )

The import popup will open up and you'll have the upload a support file ( xls, xlsx, csv or xml ) and go through the wizard.

Once the option is activated, you have the 2 options in the popup ( shown in the screenshot ) to

use this feature :

Type of opt-in mail : opt-in or double opt-in can be choosen.

Optin language :  choose the language in which the mail for opt-in or double opt-in will be send.

Once activated the newly imported contacts will be send a opt-in or double opt-in mail, to confirm there subscription.

You need to make sure your default headers are set, please check how to do that here.

You also need to make sure your preferences for Opt-in are set, check how to do that here.