Ups, I meant SLSB not a SFSB. :)
according to http://jbws.dyndns.org/mediawiki/index.php?title=FAQ#Customizing_WSDL_Schema_Generation
the only possibility you have is to use javax.xml.bind.annotation.* annotations.
Your object GetBillingAccountRequest must be JAXB de/serializable.
However aforementioned annotations don't contain MinOccurs annotation, see:
Try to ask JAXB forum how to solve your problem.