Disclaimer: I haven't tested this, so I'm not sure it will work.
I think it would just be a matter of getting the SIGAR jar and the native shared libaries into a directory that is included in the JVM's library path (which is reflected by the java.library.path System property).
So let's say you're on a Windows box and you have the SIGAR libs in C:\opt\sigar-1.6.3\sigar-bin\lib\. Then you could add that dir to your PATH environment variable in the command shell you're starting JBoss AS from, e.g.:
cd \opt\jboss-4.2.3.GA\bin set PATH=C:\opt\sigar-1.6.3\sigar-bin\lib;%PATH% run
For Linux, you'd use LD_LIBRARY_PATH, rather than PATH.
You may also be able to piggyback on the logic in run.sh/run.bat that looks for a lib dir for JBoss Native. Look for the JBoss Native related stuff in those scripts to see what I mean.
Good luck. Let me know how you make out.