Thanks for the advice about Second Succession War,Epic. I bought the PDF and looked through the ISAW rules and found them pretty interesting.
I'm not familiar with Federation and Empire. let alone the exploration rules. On the other hand I like the idea of dedicated exploration groups - they could be built like Combat Commands except the base 'Mech regiment is actually a construction group equipped with IndustrialMechs or ExplorerMechs like the Marco in TRO: Vehicle Annex, and add extra units as desired. Want to have a survey group that operates out of vehicles like the Pinto VTOL and the Chevalier wheeled tank? Buy an armour regiment. Want surveyors who operate on foot? Buy them as an infantry regiment. And so on. Then apply the modifier for the cost of JumpShips to give them transport capacity, and you're done.
For the actual exploration mechanics maybe have something like sending an Exploration Command to a hex and then rolling for the presence of a suitable star system (with a hefty modifier if there is known to be a star system in that hex, like the worlds lost in the First or Second Succession War you mentioned). If there is a suitable system then roll to see if there is a suitable world to colonise, modified by the additional units added to the Exploration Command such as infantry, armour etc (the 'Mech regiment doesn't give bonuses, it just lets the survey attempt take place), If the world is suitable for colonisation then the world can be colonised, starting as an Other World at the cost of disbanding the Exploration Command to represent the cost of setting up infrastructure and so forth. So it's nowhere near as cost effective as conquering an existing world, but if the player doesn't want to risk provoking another player then it's an option.
Also, you mentioned having to keep adding worlds to the map if players used this option. Are there any interstellar hex maps already released for ISAW?