Converting merits to CP/AN/ISP is a bad idea, because xp is dirt cheap to get in abyssea.
A better idea would be to increase CP/AN/ISP points when your xp overflows, like it does with limit points. If you get 200 xp in a conquest area, and both xp and limit xp is capped, you gain 20 (normal) + 20 (overflow) CP. Note that this only counts for normal xp, not bonus xp such as GoV pages. (Exp rings multiply normal xp, so those bonuses do count.)