I want to defrost chicken breasts and would like to marinate them. Since time may be an issue, can I place frozen chicken breasts in zip lock bag with marinade?
As long as your normal marinating time isn't significantly less than the defrosting time, then I believe that'll work fine. If the normal marinating time is much smaller, then you'll end up over-marinating your meat; if the marinade is acidic that would produce undesirable results. If it's the same or longer, then you'll be marinating for the right amount of time, and it'll be defrosted when you're done. Note that you should expect it to marinate a bit more slowly, since the outer part of the meat will have to thaw before the marinade can start doing its thing.
Jay suggests in the comments that extra water from the chicken thawing could be a problem. If it really is a large amount compared to the volume of the marinade, I guess it could be. Your options at that point would be: make a more concentrated marinade, increase the volume of marinade, or compromise and thaw partially, drain, then marinate while it thaws the rest of the way.
(I've not actually done this, and don't cook much meat, but since time is an issue, hopefully this answer is better than nothing.)