Access domains via sub-folders

[sroe]

Is it possible to create some form of "special" sub folder to enable a domain to appear within another domain.

Example
Suppose a client has two domains, abc.com and def.com. The client would like to represent the contents of def.com (filing system ..../vhosts/edf.com/httpdocs/*) as a subfolder to abc.com, so that you could access def.com via abc.com url. Something like http://www.abc.com/domains/def.com/.

When the owner of def.com approves the web site, I'd like to be able to allow access via the www.def.com directly and removed access via the abc.com url.