Mapping a domain to a subdirectory?

[Eric Longman]

Is it possible to map a domain name to a subdirectory on another server?
For example, I have a customer who would like to register www.foo.com and
have it pointed to www.bar.com/~username

Can this be done?  I was always under the impression that a CNAME, for
example, can only point to another SERVER, not to a specific directory.

Any help you can offer would be appreciated.

Eric Longman