Mm... First, you must find the rawcode of "build", then write your desired button position "x,y" in "CustomKeys..." (ah I'm forget the name) or Warcraft scripts and SLKs... (like DotA).
I hate if I forget something needed.
The files you need to replace Via import are these two
units\CommandStrings.txt
units\CommandFunc.txt
[CmdBuildHuman]
Art=CommandBasicStructHuman
Buttonpos=0,2
[CmdBuildHuman]
Art=CommandBasicStructHuman
Buttonpos=3,2
You can't host World Editor Unlimited's maps without the Unlimited WC3 patch >.<