IF you can chill the PB its theoretically feasible,
http://www.kegerators.com/articles/carbonation-table-pressure-chart.php
use the above chart to determine the equilibrium temp afaik a pb should contain 10-12psi before venting off excess via the rubber band prv.
so use 10 psi as the pressure and look up the temperature at which point that pressure will find an equilibrium at the level of condition your after.
however you will need to top up the pressure continually over a 2-3 week period since as soon as any co2 is absorbed by the beer the pressure inside the pb will drop, so it could be expensive in co2, If you can provide a constant regulated co2 supply it should be much easier to accomplish, and perhaps be accomplished in a week to 10 days.
another option would be to condition naturally in the pb, then chill down to as low as you can aim for 0-4C topping up with co2 as it chills and leave for 24 hours at target temp, then remove the top and rack of the cold conditioned beer into a fresh PB, being cold and as long as the transfer isnt too turbulant most of the co2 should remain in solution in the beer, cap the new pb and squirt in more co2, as it warms back up excess co2 dissolved should over-pressurise the PB and vent off any trapped o2
however ive not tried either so both are purely theoretical solutions..:
