To add additional information to a CD-R that already contains data, you simply write a new 'session' (a new session is created each time you write data to the CD). This will result in a multisession disc, and is supported by nearly all CD writing software.
You can't always write additional info to a CD-R disc. If the CD-R was originally written to using the 'Start multisession disc' option, then you can. When creating a CD, it is also possible to close the CD so that no further data can be written to it. If this was done, then no new sessions can be created on the CD. Most CD writing software defaults to enabling multisession and leaving the CD open, so you should be ok.
Nero,
NTI CD & DVD Maker Platinum Edition are both very good, and have downloadable trial versions. The Nero trial version, for some incomprehensible reason, does not install the Help files by default. In order to get these help files, just download and install
the English language pack. Updates to the trial version, if any, can also be downloaded from the link on that page.