Whilst there is no way of changing / updating the SMTP submission service through the Server Admin Tools as such, if you enable SSL services as a whole on your server and install an SSL certificate, this will enable SSL / TLS submission services automatically.
Please review the following articles which can be found in the Apple OS X Server documentation
https://help.apple.com/advancedserveradmin/mac/10.7/#apdE32539F0-96F0-4356-94FB-F2217E5B923D
https://help.apple.com/advancedserveradmin/mac/10.7/#apdC34BB415-E458-4C5D-9B2B-A4A92330C3C6
