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[eluser]TheFuzzy0ne[/eluser]
There are probably some fundamental difference between CodeIgniter 2 and 3. Just rename your existing ./system directory, and drop the system directory from CI 3 in it's place, and test it out. You may come across some problems that are simple to solve, then again, it might be a nightmare. CI 3 does it's best to stay compatible, but you never know. If it doesn't work out, you can just delete the ./system directory, and rename the old one back to system.

However, if this is the only feature you're wanting, then just extending your existing core will probably be the best way to go.