You can set up automation for any parameter of any effect by opening automation lanes on your track and clicking the Add/Remove button. This will bring up all your tracks, click on the track you wish, click the effect you want to automate and the parameter then just click Add. You will now have automation for your effect.
In this instance I would choose the MIX parameter on the mixverb which you can then automate, bringing it in and out as you wish in the same way as you would with Volume or Pan.
Hope this helps.