You'll have to use CB Auto Actions for that and its Format Functions. You'll need to act on the after user registration trigger with a Field action. Next under Parameters set Format Functions to Yes. Now under Action tab set the following.
Field: YOUR_DATETIME_FIELD
Operator: Set
Value:
Code:
[cb:parse function="date" date-format="Y-m-d" time-format=" H:i:s" offset="UTC"][cb:parse function="time" time="+7 DAY"][registerdate][/cb:parse][/cb:parse]
Translate Value: No
The above should add 7 days to their registration date and set it as the value of your datetime field. You can change how many days it adds by adjusting "+7 DAY". See the below on supported values as it's just using strtotime basically.
php.net/manual/en/datetime.formats.relative.php