No you can't copy
ESRIRegAddIn.exe and it's required
DADFLib.dll to another computer and run from there. Or rather, you can, but all it will do is say "addin successfully installed" without an opportunity to inspect the certificate. Tested in a virtual machine running XP and Arcgis 9.3.
From the online help for Digitally signed add-ins I see there is another utility called
ESRISignAddIn.exe that can be used to view (and add/remove) existing certificates. It is installed as part of the dot net SDK and located in
C:\Program Files\Common Files\ArcGIS\bin or
C:\Program Files(x86)\... if 64bit (ref1, ref2). You might have success using that file on another computer. Don't overlook that you'd likely be transgressing your ESRI License agreement.
As Kirk said, the addin distribution file is a just zip file with another extension. Rename to open in Windows Explorer, or use the free and open source 7-Zip which opens darn near anything without needing to rename, and have a look. None of the Addins I looked at are digitally signed so I couldn't test that route personally. (Which begs the question: what good is the ability to check certs if no one uses them?)