seterlvl is a very simple utility to set the errorlevel on the command line.

You can download the binary here and the binary with the source code here.

Here's the readme.txt:

seterlvl - Version 1.1 - Jul 19 2006
Copyright (c) 2005-2006 Ross Smith II (http://smithii.com) All Rights Reserved

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Usage: seterlvl [options] errorlevel

errorlevel can be from -4294967295 to 4294967295

Options:
-v | --version       Show version and copyright information and quit
-? | --help          Show this help message and quit

Examples:

C:\>echo %errorlevel%
0

C:\>seterlvl.exe 1

C:\>echo %errorlevel%
1

C:\>seterlvl.exe 2147483647

C:\>echo %errorlevel%
2147483647

C:\>seterlvl.exe 2147483648

C:\>echo %errorlevel%
-2147483648

C:\>seterlvl.exe 4294967295

C:\>echo %errorlevel%
-1

C:\>seterlvl.exe -1

C:\>echo %errorlevel%
-1

C:\>seterlvl.exe -4294967295

C:\>echo %errorlevel%
1

C:\>seterlvl.exe 0

C:\>echo %errorlevel%
0