Depends on your definition of "addon".
ULX is an administrative module for ULib; it was written to use ULib to allow administration of a server.
ULX has capability to have "modules", that is, extra parts, so, yes, a user could use some of it too.
Many of the "releases" in our Releases section of the forum are modules written for ULX.
ULib is our library, we wrote and intend for it's multiple abilities to be used in any type of addon or gamemode functions you wish, including if wanted, no requirement for ULX to be installed.
See
http://forums.ulyssesmod.net/index.php/topic,4633.0.html (several Lua/Ulysses resources)
See
http://forums.ulyssesmod.net/index.php/topic,3064.0.html (Discussion of our ULib docs)
See
http://forums.ulyssesmod.net/index.php/topic,3658.0.html (basic guidelines for writing code in the ULib universe)
See
http://forums.ulyssesmod.net/index.php/topic,4464.0.html ("
NEW STUFF HERE section - shows a reasonably well commented portion of writing a "ulx <blah>" command for a module.
EDIT - You changed the wording of your question while I was replying.
Regarding license of any of our work, each readme.txt file with our code contains licensing agreement.
https://github.com/Nayruden/Ulysses/search?q=license&ref=cmdformSummarized - yes, can be used, with proper credit as defined in the Creative Commons version we use, and with no profit with our derived code.