You need to give bean id name in Impex to select customFacetSortProvider
.
For Example
INSERT_UPDATE SolrIndexedProperty;name[unique=true];displayName[lang=en];customFacetSortProvider
;Brand;Brand;facetNameSortProviderAscending
facetNameSortProviderAscending
bean is referring to the "Sort by displayed name" provider. you can check this bean definition in solrfacetsearch-spring.xml file.
<bean id="facetNameSortProviderAscending" class="de.hybris.platform.solrfacetsearch.config.impl.DefaultFacetSortProvider">
<property name="comparator" ref="facetDisplayNameComparator"/>
<property name="descending" value="false"/>
</bean>
Like that based on your need, you can give the bean for the respective provider and changes will refelect in HMC as well.
Check this.