Yes, by using the <ww:set name="templateDir" value="myTemplateDir" scope="page" />.

This will set the attribute 'templateDir' in the page scope, with the value of what that is returned by 'myTemplateDir' in the stack, eg. the action class might have a getMyTheme method that return a String called template, to indicate what the template directory is within the classpath.

Another way would be <ww:set name="templateDir" value="'template'" scope="page" />

This will set the attribute 'templateDir' in the page scope with the value 'template' to indicate what the template directory is within the classpath.

See Template Loading