Currently you can't add namespaced attributes to a stanza with build(); you have to do it yourself.
Either this could be supported with a new build tag which takes three arguments (attr, ns, value); or, attr names could support being specified as something like "misc:other" where "misc" is the short name you've previously told wocky to use for the ns you care about; or, both. (The latter may already work, I haven't checked.)
Assigning this bug to me; I intend to implement this.
No I don't.