Will add in a allied tank.
Go to the units folder and go inside here: C:\Blitzkrieg\Run\data\units\technics\allies\Tanks
Copy and paste any unit you want anywhere. I'll copy the M4A1_General_Sherman_USA and rename to m4a1_sherman_polish.
The green circle is stock unit
The red circle is custom unit
The blue circle is extra custom units
Now go back to: C:\Blitzkrieg\Run\data
Open up the textures.pak
Find the units skin folder. My is (M4A1_General_Sherman_USA) so I go here: C:\Blitzkrieg\Run\data\units\technics\allies\Tanks\M4A1_General_Sherman_USA
Drag and drop the 1_h and 2_h dds files into your custom units folder (m4a1_sherman_polish)
If your running on low settings, drag and drop 1_l, 2_l, 1_c, and 2_c
Now go inside your new units folder (m4a1_sherman_polish)
Use your editing program to edit your skin.
Adobe PS dds plug-in: http://developer.nvidia.com/content/nvidia-texture-tools-adobe-photoshop
GIMP dds plug-in: http://code.google.com/p/gimp-dds/
When ready to saving in dds, this ussally pops up. I circled the options that matter.
NOTE: Some of these options appear different on updated plug-ins.
Heres my custom skin of a polish sherman.
Go to: C:\Blitzkrieg\Run\data
Open the Object.xml file inside a notepad editor.
Either at the top or bottom add this in
I added my unit at the bottom
Green circle custom unit
Blue brackets extra custom units