As well as the possibility to create and edit a playlist directly connected to an online channel it is also possible to create playlists on any other machine.

Your system administrator may have configured an offline channel which can be used to build playlists without affecting a channel which is on air. Start just:play and connect to this offline channel. just:play is now in a state in which you can create and edit playlists without working on an active (online) channel. In addition you always work in "Edit Mode".
After finalizing a playlist you can either save it directly into a watch folder of an active channel (which means the playlist is loaded immediately on that channel) or you can save it in a playlist repository folder of a channel – in this case the operator has to load the playlist on the desired position in playout.
Make sure that the time where the new playlist is scheduled is unused on your master playout when using the watch folder function. If a single frame is already scheduled, the whole playlist will be rejected and therefore not be played out. Keep in mind that only TOA XML compliant playlists can be dropped into a watch folder, other formats will be ignored.

Read the chapter "Saving and Reusing a Playlist" for more information about saving a playlist and ask your system administrator if such watch folders do exist for the particular channel(s).