The DOCX format doesn't contain definitions about the Alt and Tag properties and I'm afraid that I'm unable to suggest you an approach to save them when you are working with DOCX.
In order to guarantee that every property you are setting is exported and imported back, you could use XAML as it is the native document format of RadRichTextBox and preserves all the supported features.
Hope this is helpful.
Do you need help with upgrading your AJAX, WPF or WinForms project? Check the Telerik API Analyzer
and share your thoughts.