It can definitely be done. I may be a bassist, but the fact is that a lot of bass players have been doing similar things with their basses for some time. But you will defiinitely have to modify the guitar. The nut slots would definitely need to be widened to accomodate the heavier gauge strings. The bridge *might* need a tiny bit of modification, but you may be able to do without that. You can actually use a file to widen the bridge slots if need be.
But you'll definitely need to adjust the truss rod because there's going to be greater tension across the neck. That's just an unfortunate fact of life.
It would definitely be simpler to get a 7 string, but this type of modification is not impossible. But the thing is you need to be proficient with tools to do it. That and you would be stuck with the guitar in this state and it wouldn't be easy to get it back into a standard playing condition.
Then I would factor in the cost. If you can't readily do this yourself then you need to get a decent luthier to do the job for you. Also take into consideration how much your guitar cost. If the guitar cost in the neighborhood of $300 (Epiphone Les Paul gothic studio) - $500 and the work is going to cost you in the neighborhood of $100+? IMHO that's too much money to sink into a guitar that costs that little. Save that money and invest it in a 7 string.