With the default template of HFS 2.2 you can enter the subfolder, then click on "folder archive" on the very bottom/right of the page.
To keep the default template but to have this change of your own, you can
right click on the root/home
advanced
diff template
a dialog will popup
Enter this text
[folder]
<tr><td>%new% %protected% <a href="%item-url%"><img src="/~img_folder" /> <b>%item-name%</b></a>%comment%<td align=center><i>folder</i><td align=right>%item-modified%<td align=right>%item-dl-count%
and edit it at your wish, by adding an html link to the archive, that is
<a href='%item-url%/~folder.tar'>Download this subfolder</a>