Where should I add the necessary code to add an extra attribute to the kses allowed tags array?

In detail there’s the constant CUSTOM_TAGS that allowes defining $allowedposttags, $allowedtags, $allowedentitynames.

Then you can simply set CUSTOM_TAGS to true and define the globals in a function hooked before kses gets included.

The first available hook to fire an action that defines $allowedposttags would be muplugins_loaded, but I don’t know if kses is loaded before or after it.

// Set custom tags to override $allowedposttags, $allowedtags, $allowedentitynames
if ( ! defined( 'CUSTOM_TAGS' ) )
    define( 'CUSTOM_TAGS', true );

You also got the wp_kses_hook() that contains a self explanatory filter:

apply_filters('pre_kses', $string, $allowed_html, $allowed_protocols);