How to add to a Quicktime recording

I failed to say it is just an audio recording. So can I use QT or some other app to record and then add to it later?... I want to 'write a letter' to a friend that can not see very well and therefore can not read a letter, but an audio 'tape' would be just the ticket. But l would be creating this 'letter' over time as I think of things to say.

In that case, I would probably recommend an app like GarageBand if you already have it installed on your system and wish to use a "tapeless" workflow. It would allow you to create an audio "project" that can contain multiple audio tracks (if mixing audio) and edit/mix/control each track independently as needed. (I.e., you can plug in a quality mic, record your voice, split/move/remove voice segments as desired and stop/renew your dictation at any point.) Additional audio, music, effects, etc. can be added and/or mixed/blended with your voice narrations as may be desired. When done, the finished project can be exported/shared as an audio file, emailed as an attachment, burned to audio CD, etc. For me, the greatest advantage of this app is its "visual" graphic interface that is easy to work with once you get the hang of using it. Its major disadvantage might be offering more features than you desire to use creatively. (I.e., it is basically the audio equivalent of a video editing app.)

Is it possible to create a QT recording then come back and add to it?

Basically, the answer is "yes." The end result, however, will depend on the specific workflows involved, type of data "additions" and the software available. Generally, it's be best to start off with a high quality/low efficiency recording or one encoded in an editing specific compression format and perform your edits using a NLE, by-reference video editing techniques which allows you to create "projects" that can be continuously modified at will and export one or more target products as needed/desired for specific use.