NAS devices export filesystems. Aggregating multiple filesystems together is a non-trivial operation under most operating systems.
Assuming you can find a tool to do this, because the aggregation is at a filesystem level rather than at, say, a block level, you're going to have to contend with free space concerns in each "member" filesystem.
You might want to have a look at Dan Carley's answer re: the difference between NAS, SAN, and DAS. If you're looking at aggregation of multiple volumes into a single larger volume SAN or DAS is really the way to go for a painless process.
(I suppose, if you really wanted to do something ugly you could put some large files representing volumes on the NAS device and use Linux software RAID to stripe across the files. That's an unspeakably ugly hack... A freakish layer stack like ext3 over Linux software RAID over NFS over UDP over IP over Ethernet over the NAS devices' native filesystems and their internal RAID... ick!)