X509 Extensions

How to set Extensions for an x509 certificate, in java, using bouncycastle API?

I managed to do the "Basic Constrains" like this:

...
X509V3CertificateGenerator gen = new X509V3CertificateGenerator();
...
boolean crit;
boolean isCa;
gen.addExtension(X509Extensions.BasicConstraints, crit, new BasicConstraints(isCa));

However, I can't figure out how to do the same for IssuerAlternativeName or KeyUsage.