Although the Sims Medieval was quite a new game when you posted, every site seems to have immediately consigned it to the medieval archives. Unless there are cheats, you have to be playing a hero to improve his/her ability. If you take your time over the quests, rather than completing them as fast as possible, and do lots of XP-granting activities, your hero can reach level 10 in two or three quests.
When you complete a kingdom (i.e. use up the quest points and complete the kingdom's ambition) there will be the option to continue that kingdom in free play, and you will be able to switch heroes in that free play mode. Pirates and Nobles expansion added the options for kingdoms with free play and infinite QP from the beginning, for those not interested in kingdom ambition. These extended play modes would be suitable for someone wanting to create something magnificent like the Doomsword, which requires a lot of trading and switching of heroes.