Depending on what you want to duplicate you may be able to DirXML eDirectory to eDirectory, or if you simply want Users, Groups, Containers etc. then you can use ICE to move everything but the user passwords across. ICE can do Server-to-Server LDAP so there is not a middle step of exporting to a file and then importing the file. DSDesigner will simply represent the Tree in a GUI; it will not create it for you.
To duplicate the same NDS tree you could do a "Prepare for Hardware Upgrade" on a server holding all partitions of the tree. Copy the file to a lab server where you removed the temporary NDS. Then do a "Restore after Hardware Update" and you will have the same NDS information on a lab tree. Make sure the servers are on their own environment to prevent a duplicate tree.
Another method could be using cpqfm.nlm and to copy the complete sys:_netware directory to the lab server. Again, make sure the servers are on their own environment to prevent a duplicate tree.
Yet another approach is to use TID 10050607 "Can I split my NDS tree?". With this one you do have to be extremely careful that the duplicate tree cannot contact the production tree, even for a second, until it is renamed and any duplicate server names and IDs are changed. After that you're cool to go.
Or, if the DSMaster server has mirroring for its drives, rip one out and use it for the lab and stick in a new one in the DSMaster to rebuild. From a liability standpoint we only indirectly hint at this, but the result is the purest duplication of a production environment you can imagine. Except for the negligible risk, no real downside. Otherwise use LDIF, it's the least likely to do something bad to the production environment.