There is not a way to automatically add the donations into the Donation Thank You Letter Mailing List when importing.
If you are importing donations that all have a common criteria (like Donation date, Fund, Campaign, Appeal etc) you could create a SmartList (after you import) based on that criteria and then add the entire SmartList into that Mailing List at once.
If you do not have any common criteria that you are importing, you will want to add a column to your import file for a Donation Custom Field. I would name the column Thank You Letters then for each row, enter the word True (without the quotes) in that cell corresponding to that column.
When you import, you will map that column as a new Donation Custom Field choose the check box option.
You can then use that Donation Custom Field to create your SmartList of donations that you imported. After you create the SmartList you will need to add the contents of that list into the Donation Thank You Letters Mailing List.
To do this, click on Mailings / Manage Mailing Lists (on left side of screen) / click on the Donation Thank You Letters link under the View and Edit Donation Mailings section / Click on the Donations tab / Click on the Add Donations button / Find that SmartList you created and double click on it / Click Done and then click Done again.
This will take you back to the main Mailing Center screen. Click on the Donation Thank You Letters link on the Donation Mailings section. Follow the steps to process and acknowledge the donations.