is it possible to use
to call JBoss services? or it is used to call someting else?
if you want to stay conform to the J2EE spec, you should make yourself familiar with EJB, especially CMP and CMR. This means Container Managed Persistence and Container Managed Relationship. Using these techniques you get a Object-Relational Mapping from JBoss, which in it's 4.0 Release now officially conforms to J2EE 1.4.
For your example you will define a CMP Entity-Bean that persists to your User-Table in MS SQL. As datasource you will deploy a MSSQL-DS.xml to JBOSS. You'll find examples in $JBOSS_HOME/docs/examples/jca.
While I was beginning to enter this terrain myself I found a series of tutorials of Rick Hightower very useful. You'll find one of them under http://www-106.ibm.com/developerworks/edu/ws-dw-wscomp-i.html and the sequels over a google research. Right now they are a little outdated since the EJB-Spec went to 2.1, but they will do a pretty good job anyway. In addition you'll get free Doco on this site (Community > Documentation). Finally you should have a look at persistence frameworks like Hibernate and JDO. To understand which framework fits your needs you want come along without learning about their strategies.