There's some existing ziggurats in the cep package, cep2_core0.hak in the old 2.3 version of it I have, that might work for a starting point for modeling. nwnexplorer will let you export the model easily enough.
8096 "[CEP23] Structure: Ziggurat, Small (CCP)*" zlc_ccp_a97
8097 "[CEP23] Structure: Ziggurat, Medium (CCP)*" zlc_ccp_a98
8098 "[CEP23] Structure: Ziggurat, Large (CCP)*" zlc_ccp_a99
8099 "[CEP23] Structure: Ziggurat, Colossal (CCP)*" zlc_ccp_b01
You didn't mention in your post if you have nwmax, or nwmaxplus (or whatever the updated version of it was called) installed as well, but they're essential for making and exporting nwn models.