I'll be creating numerous sites. Each site may have default customers. These customers are content designers i.e. getting a chance to add and edit pages.

I wish to save your time by not needing to recreate these customers by hand for each new site. It is possible to method of doing this?

Yes. Make use of a shared user table:

http://codex.wordpress.org/Editing_wp-config.php#Custom_User_and_Usermeta_Tables

You need to make one site the 'main' site. Then, if you create a new site, make use of the same database, but Alter The PREFIX for every one. Then, define CUSTOM_USER_TABLE and CUSTOM_USER_META_TABLE in wordpress-config.php to ensure that it appears at the 'main' site's user tables. For instance, say your primary website is set having a database prefix of site1_, you would, for site2_, add this towards the wordpress-config.php file:

define( 'CUSTOM_USER_TABLE', 'site1_users' );
define( 'CUSTOM_USER_META_TABLE', 'site1_usermeta' );

Jons method works if you wish to share the customers whatsoever occasions, but when you want to copy them in one to a different, the simplest strategy is to produce a SQL backup from the wordpress_customers and wordpress_usermeta tables in phpmyadmin after which import them into each successive wordpress install.