array_merge
 

IS it possible for me to add custom userinfo to the fetch_userinfo() array to be then called with the fetch_userinfo() array anywhere on the site.

example i have a custom rating system based on post counts. what i want to do is do the return on it and then merge the result into the user info array.

If you add columns to the user table, they will load with the rest of the userdata and you won't have to do anything. If you use a separate table with a userid column, then you can use a plugin at hook location fetch_userinfo_query and set the variables $hook_query_fields and $hook_query_joins to add your table to the query. For example:
See includes/functions.php, function fetch_userinfo().

Edit: I just reread what you posted - if you don't want to read your info directly from the database, you could instead use a plugin on hook fetch_userinfo to do whatever you have to to find the data for user $userid, then add your info to the $user array.

thanks will give it ago and let you know :)