If by 'an already existing .html page' you mean a standard osCommerce install, then sure it's possible.
You need to modify the following file(s):
<install directory>/includes/column_left.php
- or -
<install directory>/includes/column_right.php
depending on where you want the new box to appear.
Simply add the line
require(DIR_WS_BOXES . 'mycode.php');
in the appropriate place, substituting the name of your new infobox code with the mycode.php as needed.
If you're using some template system, such as STS, then it's still possible, although adding the new code would be a little different. If you could clarify what you mean I might be able to help a little more.
If you need any other help, feel free to contact me.
Dave