Adding specific attributes if specific conditions are met

I'm currently making set bonuses, that, well, give an bonus if specific set of armor is on player. And one of the sets is Snow Ruffian, which when worn gives a specificlist of buffs, and two of them are attribute specific as far as I know: defense and speed. I need to add a value to base, so when the set is equipped - player gets +4 defense and additional 0.2 speed. I can't find anything on specifically adding to the base of attirbute, only modifying it entirely by changing the value which I don't want and don't need to do. Is there no way to add a value to attribute on specific conditions?