how chrooting a user not in his home dir

Greetings,

I'm currently installing proftpd and it works great.
Now I'd like to permit friends of mine to upload their files on my machine.

I created accounts for them but I do not want them to upload their files
in the default /home/user dir (let's say the partition is full )
but on another disk which could be like this :

/customers/user1
/customers/user2
/customers/user3
And so on.

I tried to create anonymous sections to the differents dir but any valid
email can enable the user to access the dir (of course ).

I'm Googling but found no help yet

any tip, link, tutorial is welcome

Many thanks !