To start with the EFI chip its self is also a factor here I do believe this series had a 32bit chip.
As to the EFI code on the chip you don't need to alter it if you use a Thunker which is a shim service which intercepts the calls and converts them from 64 bit to 32bit and back. There are limits here as the API call the app is calling may not have a 32bit version. In that case the Thunker needs to also offer routine that will fill-in the needed code for the lack of that given API.
OK, where Am I going here? There are thunker's for OS-X to allow newer versions of OS to work on older systems. Apple in fact had one in Mountain Lion (abet a very simple one) so developers could migrate there apps to 64bit. Apple also allowed dual 32bit & 64bit app support for quite a few years. But there are limits here! Apple forced all of the developers to remove the 32bit code from their Apps if they wanted to still be on the App Store.
Sadly, your system is just not powerful enough to support macOS Sierra. Between the CPU's limits you have RAM & drive limits. You can only access 3 GB or RAM and the systems SATA port is only SATA I (1.5 Gb/s). You will be hard pressed to get anything but the simplest app to run on such a limited system. Don't even think you can task switch between apps or access the newest features the OS offers. Don't forget you'll loose access to your older OS-9 apps as Rosetta services are no longer offered in the newer versions of OS-X.
OK, so maybe I can't run the very newest OS can I at least get a little more time out of my system? You could give this Thunker service a try: MacPostFactor I still would caution you not to expect this as a cure all it might buy you a bit of time. I would first try Mavericks to see how that ran.
One last issue here: Your system also has some hardware flaws (GPU & battery) that will over time catch up with you so do start saving your pennies for a newer system (new or used).