Who said the disk was formatted? If the disk was finalized when the
previous content was added to it, there's no way to add more. In other
words, if packet-writing wasn't used, a CD-RW (or DVD-RW) is just like a
CD-R, except that it can be erased.
The OP can copy the contents of the CD to his hard drive, erase the disk
(or format it using packet-writing software) and then copy the files
back to the CD. If packet-writing isn't used, the disk should not be
finalized if you want to add more files later.