I think you cannot put any java specific return type on any web service operation, because web sercies are all about interoperability think about some dot net client want to access your web service? they might have diffrent implementation for HashMap or so...
The problem is that there is no representation of maps in an XML schema.
You can, however, translate the map to something that can be represented using an array and a POJO.
The array is an array of POJOs an each POJO instance holds a key and a value.
The key and value are of some simple type, like String, Number or similar.
Here is a code example:
Note that this will not readily be transformed to a map, so the client has to have some additional information, apart from the WSDL, as to how to treat this data.