There was a bug in EGM 1.086 that meant the Omega 4 relay would sometimes not be accessible. It was fixed in the Beta, so it should be available any time after priority:palaven is complete. Adding new star systems is a difficult process to explain quickly, but a good start is to get ME3Explorer and start playing around with modding the files https://www.nexusmods.com/masseffect3/mods/409. There is a good wiki http://me3explorer.wikia.com/wiki/ME3Explorer_Wiki , forums and discord chat as well. As for Alpha Centauri - I believe this mod https://www.nexusmods.com/masseffect3/mods/634 has you covered - plus lots of other systems from the extended lore.