I played lap-style on a round-neck with an extension nut adapter for about ten years before finally getting a squareneck. I used GBDGBD tuning with no problem.

The only string in GBDGBD tuning that's tuned appreciably higher than "standard" is the low string (G instead of E). If that's a concern, swap it out for something a bit lighter.

The real problem with playing an adapted round-neck is that most of them will never sound anything like a squareneck. The vast majority of round-necks have a "biscuit" bridge that rests on the center of the resonator cone. Squarenecks have a "spider" bridge, which is essentially a set of 8 metal "legs" that rest on the outer rim of the cone. The tone of a biscuit bridge reso is a bit muffled and generally considered suitable for bluesy styles while a spider bridge gives a more open and brassy sound suitable for country and bluegrass styles.