The way the default terms are added to a vocabulary is not so nice indeed, and we get to hear this kind of complaints often.
But, there is a module which is made exactly for this purpose :)
Check Taxonomy CSV import/export.
This module allows you to import or export taxonomy from or to a CSV (comma-separated values) file or with a copy-and-paste text.
It helps you to quick import a non-standardized vocabulary, for example an old thesaurus, a hierarchical taxonomy or a simple list of children, synonyms, descriptions, weights or related terms to terms. It can be used as a module or as an API with a plug-in mechanism. It can manage internationalized vocabularies with i18n_taxonomy, a submodule of i18n module [Drupal 7 only].
After that you just have to train the client on how to import the terms. The terms can be added in a single configuration textarea and can be comma separated.