I've built with an outter sleeve on tennessee grip rod, no problems there. Now I've got a split grip project that needs to add 3" to the rod and I don't have any stock large enough to sleeve over the outside of the rod, but I do have enough to sleeve the inner portion. Is this a smart thing to do? The outter sleeve, I'm confident, will never come undone. But inserting an extension inside the butt end of the rod just doesn't seem entirely correct. Is there a better way to do this? I'd like to save weight, but don't want to risk any adhesive failure.

It's not the best way, but for that type rod and the expected loads it'll experience, and coupled with the slight amount you're needing to extend it, you can likely do just fine with an inner sleeve. Adhesion isn't the issue - keeping the extension as strong and stiff as the butt section you're inserting it into is. But again, for such a short length this isn't likely to cause any problems.

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Just prep the areas to be glued well and you will be fine

Bill - willierods.com

For good measure I'd overwrap the outside with thread

Andrew.
Go and find a piece of blank that would fit over the current blank.
You can go to another rod shop, find an old broken rod, go to the good will or salvation army and pick up a blank etc.

Or you could go with the inner blank repair.

Roger