How to extend ICU collation with custom rules in SQLite?

I want to use ICU collations in SQLite for sorting the results of SQL queries. I have customized collation rules which I want to load on top of the default rules for a specific locale. So I first want to load the collation for a locale and then I want to load the customized collation which should extend, but not replace the previously loaded collation.

How can I achieve this with SQLite?