I have done this with external hard drives but not with a thumbdrive. I can't see why it would work on one and not the other.
This is the program I have used :
Mediafour - MacDrive - Access Mac Disks | . Not a free one but it works.
You may want to take some time to read the website to be sure it will do what you want. There is a 5-day free trial period.
EDIT: (Clarification): I had the opportunity to use Mac Drive last night. Owner wanted to delete the Windows partition and use the entire LaCie external hard drive for Mac files.
Mac Drive can not format Windows partitions or create HFS+ partitions from Unallocated Space.
The procedure would be to delete the partition creating unallocated space using Windows, and then use a Mac computer to create the MAC partition and format it HFS+.
In addition: most flash drives can not be partitioned without hacking them to not be recognized as Removable Media, so it is not a straight-forward, simple procedure at all.
http://www.ghacks.net/2009/04/17/partition-usb-flash-drives/ . But even this is an XP trick and may not work with W7.
For your "partitionable" thumb drive you would use Disk Management to create a Windows partition, leaving the remaining space as Unallocated.
