You would need to bring a new electrical circuit from the Genset transfer switch distribution panel to plug the APC UPS's into, and bypass the liebert.
The tech was correct when telling you that anomalies can occur when a UPS is plugged into a UPS, and if you are concerned with your servers the prudent thing to do is to supply the best power supply to them with the redundancy of multiple UPS's backed up by a Generator.
http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_sid=uz6B5zsh&p_lva=&p_faqid=397