Extensions have long been written for OpenOffice.org Writer. However, the fact that attention is finally being paid to other applications seems a sign that OpenOffice.org is finally starting to develop an active extension-writing community.
A case in point is the recent availability of extensions for Impress, OpenOffice.org's slide show program. Extensions for Impress are still outnumbered four or five to one by those for Writer, but at least they are now being written. Just as importantly, they are filling important gaps in functionality, and encouraging uses for slide shows other than the standard presentation in work or education.
All these extensions are installed in the normal way -- that is, you download their files, and then use Tools - > Extension Manager from within OpenOffice.org to activate them. If you are installing them just for the current account, then select My Extensions before navigating to an extension's file to install it.