Sorry, you can't change the home directory nor is it a good idea to do that.
What you can do is login as each FTP user and make a special directory - name it "wp-uploads" for example.
Then go to the Web Panel and choose Domains -> Remap Sub-dir
First you indicate the URL - [example.com] / [blog/wp-contents/upload/username]
On the next page you indicate the file system path: [/home/username] / [wp-uploads]
And when you submit, the web server will be setup to look for files for http://example.com/blog/wp-contents/upload/username in /home/username/wp-uploads
So then you tell the people to upload the files to their wp-uploads directory.
Alternately you can use symlinks links as well if you are familar with using a shell. You'd have to turn off extra web security in the Domains -> Manage Domains panel.
Customer since 2000 openvein.org