Unless you know how to do an XSL transformation with your XML import file, unknown fields cannot be correctly mapped onto an existing MultiTerm termbase definition when importing them (e.g. import from another non-SDL termbase).
Either no terms or inconsistent terms/fields/values (violating the termbase definition settings) are imported. As a workaround, you may use Excel/text editor and convert it MultiTerm XML with tools such as Glossary Converter. However, I would like to see an import option/wizard to correctly map fields according to the termbase defintion without the use of XSLT for the end user.