Okay, let's walk through this using the built in ATSF SD40-2,<kuid:-1:100871> since that's already build 2.9.
Select in content manager, right click, select Clone.
Since it's a clone of a built in you'll get a bunch of errors, so right click the clone and select Open With, Images2TGA.bat. If you don't already have Images2TGA;
http://members.optusnet.com.au/~villaump/pevsoft.htm
Download, install, configure, you should have Images2TGA, PM2IM, and Mesh Viewer if you want to get into reskinning anyway.
After Images2TGA is done close it, open the new SD40 in content creator. That will automatically delete the obsolete name-it tag. You can change the username while you're in here, then save and exit.
Back to content manager, edit, commit, check for errors.
Now download CNW F7A 1,<kuid:522774:100404> as this will be your template to steal all the ARN stuff from. Set it up so you have both your new clone and the CNW F7A in the main list, open both for edit in explorer. Copy the fx_loco.gs file from the CNW F7A, paste into the folder for your SD40. Open both config.txt files, move and resize the notepad windows so the CNW F7A is above to avoid confusion.
Copy the two lines script and class, paste into the SD40 config in the same location, right under the interior tag.
Scroll down;
Just above the mesh table copy and paste the datablock for the extensions. Scroll to the bottom;
And copy the arn_lib and sfx_lib and insert them into the SD40 KUID table. Save the config.txt, commit, check for errors, launch game and see if you have auto numbers.
Note here for the extensions I used
number_low "4000"
number_high "4200"
You can change those to whatever number series you prefer.