Unfortunately, it is not possible to import an external wav or mp3 file into Zencastr at the moment. This is something we would like to add but no release date just yet.
If you find that you have two separate tracks for a participant due to a recording drop-out or error, then you will need to put the two different tracks together in a separate audio editor such as Audacity or Adobe Editor etc. Zencastr is unable to do this at the moment. Once you have the two different tracks merged into one track, you can then join your track and their track together in that same audio editor.
If you need further clarification on this, please get in touch with support.