You can define AWS access keys in your wp-config.php file and use them for any S3 remote library.

define('AWS_ACCESS_KEY_ID', '********************');
define 'AWS_SECRET_ACCESS_KEY', '************************************');

If you want to define more than one set of access keys you can define them by adding an increment at end of the constants except for the first set. Like this example:

define('AWS_ACCESS_KEY_ID', '********************');
define 'AWS_SECRET_ACCESS_KEY', '************************************');

define('AWS_ACCESS_KEY_ID_1', '********************');
define('AWS_SECRET_ACCESS_KEY_1', '************************************');

define('AWS_ACCESS_KEY_ID_2', '********************');
define('AWS_SECRET_ACCESS_KEY_2', '************************************');

Once a set of keys is defined in your wp-config.php file. A select menu will appear to offer you the option to use your defined constants or the library access keys input fields.

Now, if you define multiple sets of access keys, the select menu will offer you the option to choose the constants set you wish to use for the current remote library.