Help Builder is based on the Visual FoxPro table format and thus can be fully extended with custom fields of your own. To add a new field use the Fields... editor in the Properties Dialog. You can simply add a new field to the table and use those new fields in the HTML templates like this:

<%= oHelp.oTopic.NewFIeldName %>

or

<%= oHelp.FormatHTML( oHelp.oTopic.NewFIeldName) %>

Furthermore you can edit new character and memo fields directly in Help Builder's IDE. When the structure of the table is different than the base file (__wwhelp.hbp) a new Custom tab is added to the Edit Topic view. The tab consists of a drop down of all the custom fields and an edit box that lets you edit the values for each of the custom fields.

Keep in mind that all fields must be character in order to be editable here. Non-character fields will work, but will format unpredictably and are not recommended.