You could link one of the star systems to the sun--and ships could travel from that sun to the other suns, but if those other suns aren't linked to anything, it would be kind of silly to jump from the stars. What Harpo said is the way to do it.
All of that having been said, if your goal is to make a map that will be played in online multiplayer PvP ("real Sins games"), don't bother. Most people who play PvP would prefer to play on single star maps and/or wouldn't take the effort required to add your map to their Sins game and custom maps (made in Galaxy Forge) do not auto-download.