<p>I'm trying to create a build script for my current project, which includes an Excel Add-in.  The Add-in contains a VBProject with a file modGlobal with a variable version_Number.   This number needs to be changed for every build.  The exact steps:</p>

<ol>
<li>Open XLA document with Excel.</li>
<li>Switch to VBEditor mode. (Alt+F11)</li>
<li>Open VBProject, entering a password.</li>
<li>Open modGlobal file.</li>
<li>Change variable's default value to the current date.</li>
<li>Close &amp; save the project.</li>
</ol>

<p>I'm at a loss for how to automate the process.  The best I can come up with is an excel macro or Auto-IT script.  I could also write a custom MSBuild task, but that might get... tricky.  Does anyone else have any other suggestions?</p>

I'm not 100% sure how to do exactly what you have requested. But guessing the goal you have in mind there are a few possibilities.
1)
Make part (or all) of your Globals a separate text file that is distributed with the .XLA I would use this for external references such as the version of the rest of your app. Write this at build time and distribute, and read on the load of the XLA.
2)
I'm guessing your writing the version of the main component (ie: the non XLA part) of your application. If this is tru why store this in your XLA? Why not have the main part of the app allow certain version of the XLA to work. Version 1.1 of the main app could accept calls from Version 7.1 - 8.9 of the XLA.
3)
If you are just looking to update the XLA so it gets included in your version control system or similar (i'm guessing here) maybe just touch the file so it looks like it changed.
If it's the version of the rest of the app that you are controlling i'd just stick it in a text file and distribute that along with the XLA.