Do you mean you can't do it, or can. Because, if you meant can, that smiley attempting to splatter his brains doesn't exactly make sense!
I'll assume you mean you can't, and I'll try to help you out a little bit with what I know and tried...
As Corbo said:
QUOTE
===The tools you need===
Retro GRP
WinMPQ
MPQDraft
Image Editor (any, Even paint is useful)
RetroGRP will take the .GRP file containing the unit animations and turn them into many .BMP's which you can edit in any image editor i.e. Photoshop or even MSPaint.
WinMPQ is what you will allow you to modify the .MPQ, lists of SC data files. You can play that mod using MPQDraft, which is a self-extracting patch maker.
Now that you know that...
We will use WinMPQ and open the StarDat.MPQ file in the starcraft folder on you computer, scroll down to units/.../... and find the .GRPs of the units you would like to combine. You will want to extract those .GRPs to your mod folder.
Make sure you know what parts of each unit you want to combine and which .GRP the final project will be substituted for.
Open RetroGRP where it says "Load External File", find where ever you put the extracted .GRP and double-click it. Where it says "Set Output Directory", you want to put the folder or location you want the .BMPs to go. Once you have all of those, click "Write out BMPs" and then, in the pop-up click "All BMPs".
The program will turn the animated GRP into a series of BMPs which can be edited. Be warned though, may of the .GRPs will contain over hundereds of BMPs which each have to be seperatly edited.
Do the same process for the other .GRP(s) that you want to combine with the first.
After all that is done, combine the specific parts of each into each .BMP, open the .BMP of the unit you want to have the image substitute for first so when you hit save, the edited filed stays in it's original spot.
After you have edited the image, open up RetroGRP again. This time we will be combining the .BMPs into a .GRP which can be put into a mod. Set the output directory to the folder where your mod is and click "Compile to GRP". Then, select the .RAL file of your edited unts. RetroGRP will do the rest.
Put that file into WinMPQ and make sure to remove the "X_" part of the file name. Run the mod in MPQDraft and viola! A combined unit image.