To add/include domain:
stsadm -o setproperty -pn peoplepicker-searchadforests -pv "domain:newaddom1.corp;domain:newaddom2.corp;forest:newaddom3.corp" -url http://portalsite:

To retrieve/view existing domain:
stsadm -o getproperty -pn peoplepicker-searchadforests -url http://portalsite

Remember to perform IISReset after add/include new domain.