The difficulty would not be converting spell slots to points, that calculation is extremely simple. The complications that come in are balance concerns. A pure sorcerer is fairly underpowered in comparison to a wizard, but when multiclassed his primary weakness is ameliorated. Further, if a level in sorcerer gets you spell points in lieu of slots, most casting classes would probably want to get a level of sorcerer to utilize that system even if they aren't interested in actually being sorcerers. Spell points offer more power and versatility than spell slots, simply put.