you mean use a harddisk through an adaptor into a RAM Slot?
don't think one exists, and doubt it ever will, harddisks are too slow.
And you can use disk space as psuedo-ram, it's called swap (windows uses anywhere on a disk, linux uses a dedicated partition for it)
You can, however, use RAM as a harddisk. like on of these:
http://www.newegg.com/Product/Product.asp?Item=N82E16815168001
(perhaps this is the "adaptor" you're thinking of...)