g1awrapper creates a header on binary executables.
./g1awrapper <binary file> [-o <output name>] [-i <icon image>]

Example run:
sh-3.2$ ./g1awrapper addin.bin -o myaddin.g1a
-- g1awrapper v0.1 Copyright (C) 2008 Andreas Bertheussen.
[>] Reading default icon (icon.bmp)..
[I] Filename 'addin.bin' selected as binary source.
[I] Filename 'myaddin.g1a' selected for G1A file (set by user).
[I] Timestamp: 2008.0222.0316
[I] G1A size: 512
[>] Writing header.. OK.
[>] Writing data.. OK.
sh-3.2$

Download and run

Get the source code (and default icon.bmp) from SVN. You could use TortoiseSVN for this if you are running Windows.
Now use qmake to create a new project and makefile, then run make;