That shouldn't be a problem for you - there's an option in the software to "copy back" to Exchange. This will push the contents of messages that have been archived back into Exchange, just as they were before you installed.
The only thing that is worth bearing in mind is that when this happens, Exchange is unlikely to re-"single-instance" messages that have been sent to multiple people, so the information store may end up larger than before archiving. If you're running up against the SBS size limit, you might want to consider this before doing so.
There is also the option to run the archiver in "copy entire message" mode, which won't actually remove any information from Exchange, but rather just copies it to the archive. Of course this means that you don't need to copy back if you wish to uninstall, but equally you won't see the size reduction in Exchange in the first place, so it may not be so useful to you.
If you'd like to see a demo of the software in action before installing it yourself, please let us know - you'll find a "live chat" button on the Exchange Server Archiver page
Any other questions, please don't hesitate to ask.