Go on camp menu and select the "Import/Export NPC" feature, select the companion you wish to change and press "C" then click on "statistics" on the bottom left corner of the character screen and select "export". Minimize your game window, go on "documents/mount&blade warband/characters. There will be a .txt document for each character you do this for (including your own, which you can do through the same process but from the "character" menu). Open the .txt file and you will be able to change all stats for that character.
In my current playthrough I gave Ser Barristan an equally matched proficiency in double-handed weapons and polearms, as he is the Lord Commander of the Kingsguard, a warrior of great renown and prestige, and he has a solid record in tournaments on horse and lance even into his old age at the start of the books story. The other proficiencies I gave him a decent, not bright but not piss poor either, as I doubt he had been trained only in single-handed weapons, and, as you pointed out, he would likely shoot better than the average peasant.
