Those screenies are taken from old versions of PwnageTools. PwnageTool no longer allows you to enable base band. It's greyed out. I've made your Custom Firmware with Activate Unchecked as you requested. Shouldn't that work?! Then you activate with your AT&T sim?!?
Yes, that will work... Thank you very much..
So the stupid guide below is outdated.. I got confused because I was also under the impressing that pwnageTool doesn't update the baseband anymore...
http://www.xsellize.com/showthread.php?t=67502
EDIT: Please let me know when you done uploading