OK here is the unofficial steps how to get it work, but no guarantee what it will do to the system:
So the only way is to install the unpacked extension in Developer mode:
1) First you have to get the unpacked extension
a) either copy the SPAUTOFILL.CRX to SPAUTOFILL.ZIP and extract the content of the ZIP file to some folder
b) or locate the installed extension in the default Chrome user profile (C:\Users\<UserName>\AppData\Local\Google\Chrome\User Data\Default\Extensions\offlaklpbgccmeobfnimdjapgolbfhad\<VersionNumber>\) and copy the content to some folder
2) Enable the Developer mode in Chrome by checking the "Developer mode" checkbox on the Extensions page
3) Install the extension by clicking the "Load unpacked extension..." button and locating the folder where you have placed the extension in step 1)
Unlike the case of installing the extension via CRX file, where the extension is actually installed/copied to the user's profile, the unpacked extension is only loaded from the original location (i.e. it's not copied anywhere).
So actually just pointing to the default profile location in step 3 should work too...