If you edit your SHOWTHREAD template... find the line:
Code:
<title>{vb:raw thread.prefix_plain_html} {vb:raw thread.title}</title>
You can add words immediately after <title> or immediately before </title> to add words to your title (either before or after the auto created title.)
Example
Code:
<title>Thread View - {vb:raw thread.prefix_plain_html} {vb:raw thread.title} More Keywords Here</title>