The majority of storage systems can be added by simply entering the IP address for the target in the "Portal/Group" field, but the newer QNAP firmware (since 4.0.2) seems to require that targets now be added using the specific target iqn.
Rather than using "Portal/Group", please use the "Target" option when adding the target in globalSAN, and then manually enter the iqn for the QNAP LUN in the "Target Name" field, followed by the IP address in the "Connections:" pane on the right.
This is current as of globalSAN v.5.2