No you can't buy Practice Points. I am guessing the reason is it would upset the balance of the game since these are the Points that your Pirate uses to train in extra Powers and Talents. Each Pirate can only have so many Powers and Talents by Practice Points. Of course, gear and Pets (that grant a Power or Talent) can allow you to have more Powers and Talents.
In case you didn't know, your Pirate gets one Practice Point when you reach the following levels: 8, 12, 16, 20, 25, 30, 35, 40, 45, 50, 55, 60, and 65 (Level Cap at the moment). You can also get one Practice Point for each of the quests that Prospector Zeke sends you on when you complete the quest.