Reply to when and if's in mapping?

hey guys hope you can help me. I believe there has to be such a thing so help me find out where ^^

I'm mapping my M-Audio Mixlab right now and i want one of the encoder to do two things. First i want to select a effect with it. when i found my effect i want to press the effect on button and my encoder should then change the effect amount. When i turn the effect off, the encoder should again select the effect.

Would be happy if you could help me