currently not possible. imscp can not operate in dmz as nameserver for external resolution.
use external nameservers, disable imscp local resolver.
workaround could be (but not suggested) to modify your zone templates, so zones are created with external nameservers instead of nsX.domain.tld
how to do that is described in multiple topics on our forums.
managing zones and nameservers is already on todo list, imscp will support that in near future with a plugin.