How to Create a Custom Slug for Tags and Categories with a prefix or suffix?

Filter 'term_link'. Sample code, not tested:

add_filter( 'term_link', 'wpse_72848_change_tag_slug', 10, 3 );

function wpse_72848_change_tag_slug( $termlink, $term, $taxonomy )
{
    if ( 'post_tag' !== $taxonomy )
        return $termlink;

    if ( 'product' === $term->slug )
        return str_replace( '/product/', '/product-powerpoint-ppt/', $termlink );

    return $termlink;
}