Probably impossible, the joint would likely end up to weak due to the wood removed. Even if you managed it, you would need to increase the size of the pickup cavity for each pickup to accomodate a 7 string pickup, AND you would need a new bridge, which would require filling in the old holes and drilling new ones for a TOM bridge and something dam near impossible if you were changing a 6 string floyd to a 7.
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Do you know how many people ask this a week? No you can't change a 6 string into a 7 string, you'd be better off just buying a 7 string by the time you've spent all the money on a new neck and pickups and hardware.

"Show me war; show me pestilence; show me the blood-red hands of retribution..."
If you honestly want the low end of a 7 string, you can sacrifice your high E and put a 7 string set on it.

Other than that, no, the only way your getting a 7 string is getting a new guitar.
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You have a walnut stop sign banjo-tar signed by MAB

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You deserve an Awesome Award for Awesome People.

Stop Sign Guitar? HELL YES!