I can't imagine the Federation would force Membership upon anyone, nor can I imagine that the Federation would accept as a Member State any state which has conquered a foreign population. The UFP would almost certainly demand that any new potential Member State liberate any conquered worlds before joining the Federation.
That would surely depend on how much the subjugating world has to offer as a member of the Federation. Politics is the art of compromise.
I imagine the subjugated worlds would count as three-fifths in determining representation on the council.