I would probably stay put and decline the trade.
I don't think Alonso is going to be a keeper in your league this year or the next. So giving him up isn't a problem IMO. You could always redraft him at some point. A 7th round pick ends up being an 11th round pick with the 4 keepers. Not a great pick, but not a horrible one either.
Your keepers are Kemp, Holliday, Haren, and Posey. Yet you aren't keeping Pablo? I'm not a fan of keeping most catchers in a 12 team league, especially Posey. His injury last season should make him easier to redraft than the other 4. I'd drop him for Pablo on the list. And with Pablo, I think he is just as valuable as Zimmerman going forward. Zimmerman has a tad more talent IMO, but is much more of an injury risk.
If you make the trade, the guys I would keep are the 4 hitters- Kemp, Holliday, Pablo, and Zimmerman. Haren is bumped off the list- not a lot of teams and the other guys are being drafted higher than Haren. And keeping Zimmerman (or Pablo) instead of Haren isn't enough of an upgrade to lose an 11th round pick.