So the answer to this is yes you can...but it's very much "not out of the box"!
Here is a method on how...
First you'll need to get the Long/Lat for your Zipcodes (there are free ZipCode Lookup tables...just search google, usually provided by government or census websites). You'll need to do this as you can't access the "Generated" (by Tableau) Long/Lat in formulas (which we need to do here). Also as you want different radius for each point, your set up will be a little different (the above link gives blanket radius circles for each point).
I have actually just used this method to build a catchment area analysis model...so I can verify that a) it works and b) is quite complicated!
Hope that helps?...please post back if you have any issues adapting Richard's solution to your situation (ideally with some dummy/representative data) and I should be able to help out.