If your question if like this:
Category A
– sub-category 1
– sub-category 2
– sub-category 3
Then you would create the following in your theme’s functions.php:
//create the main category
wp_insert_term(
// the name of the category
'Category A',
// the taxonomy, which in this case if category (don't change)
'category',
array(
// what to use in the url for term archive
'slug' => 'category-a',
));
Then for each sub-category:
wp_insert_term(
// the name of the sub-category
'Sub-category 1',
// the taxonomy 'category' (don't change)
'category',
array(
// what to use in the url for term archive
'slug' => 'sub-cat-1',
// link with main category. In the case, become a child of the "Category A" parent
'parent'=> term_exists( 'Category A', 'category' )['term_id']
));
Hope this helps.