A list of players is displayed when left clicking the volume indicator applet of Xfce panel.
But others are installed that I want to see there. Can I add them?
Yes there is. You need to run Dconf Editor. (This is not limited to Xfce.)
As specified by Takkat in a comment, this will fully work only for players controlled by MPRIS and DBus. This type of players will automatically be listed if installed. (This means that dfconf will not be useful to add such programs, as they are automatically added, but only to set the preferred ones, with the play/pause and next/previous buttons, and to remove unwanted players by adding them as 'blacklisted'.)
A sort of workaround for "unsupported" players like DeaDBeef or SMPlayer is to set them as 'preferred', which will display the play/pause and next/previous buttons. The buttons will not work as intended for these programs, but the players can be started with the 'play' button.
In the image below, for ex., the unsupported DeaDBeeF (as well as the supported Audacious) can be started, but SMPlayer cannot. After adding smplayer in the list of 'preferred', a big 'play' button appears (after logout) which can be used to start the player - yet not for play/pause the media.
(Log out and in to activate the new settings.)