I believe you have several user roles on the site where you have imported these users from. Which is why they don't have any role now.
You need to do this:
1. Choose the role you will be granting.
Open ow_base_authorization_role database table. Here you will see list of your roles with their IDs. Remember the role ID you wish to give to all your users. In our example we will remember role ID = 12 ( which is usually a default free user role that comes with standard SkaDate / Oxwall installation).
2. Grant selected role.
Open ow_base_authorization_user_role database table.Click “SQL” tab in your PhpMyAdmin interface.Insert following query:
INSERT IGNORE INTO `ow_base_authorization_user_role` (`userId`, `roleId`) SELECT `id`, 12 FROM `ow_base_user`
Where 12 is the role ID which will be granted to all your users.
And you are done.