Is there a way to control the Numa settings on my iMac?
Does Lion even support NUMA ?
For Snow-Leopard, I thought, there was no NUMA support. German article as source.
I could not find a source for NUMA support in OS X, thus I would say that there's as well no way to control the NUMA-settings ;)
This Intel document states:

On multi-socket machines based on the Intel® Core™ i7 processor, NUMA is typically available as a BIOS boot option, but effective use also requires some OS and some user-level intervention.
Windows* OS support for NUMA started with Windows* Server 2003, and Linux* OS support requires kernel 2.6 or higher.

No OS X mentioned.