You can press the REC button whilst recording to pause the recording, then press the REC button again to continue to recording. This is the only way to add to a file whilst recording - you cannot add to it if you have already pressed STOP.
To concatenate recorded files on the PC, you can use the simple free online tool: https://audio-joiner.com/
or you could use one of the many free merging programs available for download: https://www.shchuka.com/software/mergemp3/
Let me know if you have any further questions on this :)