In order to create new datasource programmatically on JBoss 7 without restart (on the fly) from Java you can use CLI Java API. You have to include jboss-as-controller-client to your project dependencies:

<dependency>
	<groupId>org.jboss.as</groupId>
	<artifactId>jboss-as-controller-client</artifactId>
	<version>7.0.2.Final</version>
</dependency>

The following example creates new datasource:

public void createDatasource() throws Exception{
	ModelNode request = new ModelNode();
	request.get(ClientConstants.OP).set(ClientConstants.ADD);
	request.get(ClientConstants.OP_ADDR).add("subsystem",
			"datasources");
	request.get(ClientConstants.OP_ADDR).add("data-source",
			"java:jboss/datasources/NewDatasource");

	request.get("jndi-name").set("java:jboss/datasources/NewDatasource");
	request.get("connection-url").set("jdbc:as400://1.2.3.4/SCHEME");
	request.get("driver-class").set("com.ibm.as400.access.AS400JDBCDriver");
	request.get("driver-name").set("jt400.jar");
	request.get("user-name").set("username");
	request.get("password").set("password");
	request.get("pool-name").set("pool_NewDatasource");

	ModelControllerClient client = ModelControllerClient.Factory.create(
			InetAddress.getByName("127.0.0.1"), 9999);
	client.execute(new OperationBuilder(request).build());
}

If you want to check if the datasource already exists, consider following snippet:

public boolean checkIfDatasourceExists() throws Exception {
	ModelNode request = new ModelNode();
	request.get(ClientConstants.OP).set("read-resource");
	request.get("recursive").set(false);
	request.get(ClientConstants.OP_ADDR).add("subsystem", "datasources");

	ModelControllerClient client = ModelControllerClient.Factory.create(
			InetAddress.getByName("127.0.0.1"), 9999);
	ModelNode responce = client.execute(new OperationBuilder(request).build());

	ModelNode datasources = responce.get(ClientConstants.RESULT).get("data-source");

	if (datasources.isDefined()) {
		for (ModelNode dataSource : datasources.asList()) {
			String dataSourceName = dataSource.asProperty().getName();
			if (dataSourceName.equals("java:jboss/datasources/NewDatasource")) {
				return true;
			}
		}
	}
	return false;
}

The examples have the same effect as trying to add datasource from Jboss Administration console.