User import calls custom field endpoints of mod-users to find out the list of available CFs and create/update fields if required. This is done now by using HttpClient with raw requests. Custom fields model is partially copied from folio-custom-fields. This adds additional integration complexity because the model should be kept in sync with the original one but it can be achieved only by manual actions.
Meanwhile folio-custom-fields is going to provide a java client for its API (see FCFIELDS-7) and a separate artifact with API interfaces and model (see FCFIELDS-6). Once the client and the artifact are available they should be imported by mod-user-import and used to replace bare HttClient calls and the schema duplication