We use a customization script for this. it works for us, and also solves the problem about accumulating time by type of activity (i.e. how many QA hours, compared to developers, or to BA or doing reviews).
One inbound email can result in changes to multiple areas of programming. We have a process, where we create one sub-case per feature or area. within each 'feature' sub-case, we create sub-cases for BA, design, development, review, qa test runs and implementation. All the development time rolls up into the development sub-cases ,and marked with an area and category as such.
we added some js to create all the sub-cases according to our process. it uses the API and works really well. being new to Fogbugz as we were, it was costly to make; and I wish this or something similar was just part of the product.
It was originally sparked by my questions to FogCreek about qualifying time during timesheet entry e.g. I spent 2 hours doing xxxx kind of activity (e.g. dev, reviews, qa testing etc...). that would have simplified this whole thing for us.
if you are interested, I could show it working to you (.mp4?), but to post the code I would have to clean it up a bit first. i.e. have time
Mike
Here's a link to the .mp4 click here