If you need several indexes to list other sets of terms, you will use user-defined indexes. But this kind of index cannot combine identical entries, and does not sort them alphabetically.
Extension User Index Sorter will sort the user-defined index tables of a Writer document (but will not combine identical entries). You can select which tables to sort, and it will be remembered in the document. The extension adds two menu items:
- Menu Tools – Add-ons – Sort Options For User-Defined Indexes...
- Menu Tools – Update – All Indexes And Sort User-Defined Indexes
User Index Sorter is downloadable from Apache OpenOffice Extension Repository.