Hi,
Yes, JoomSEF can be configured to add trailing slash to all generated SEF URLs. However, this works only for SEF URLs handled by JoomSEF, so if you are using some component for which there's no JoomSEF extension available and its SEF URLs would be handled by Joomla's default router, they won't have the trailing slash added. Your SEF URLs would also change, because JoomSEF generates them differently than Joomla.
The feature is available in free version of JoomSEF, so you can test it before purchase. In order to have trailing slash added to SEF URLs, set "File suffix" option to "/" in JoomSEF's Configuration on the Basic tab and make sure that "Remove trailing slash" option is set to "Never" on Advanced tab.