well back in the day traders pretty much went anywhere they liked. Sure they weren't citizens but there's a difference between not letting someone in, and not letting them be a citizen. Obviously some places are citizen only, like the temple. But back in the day you pretty much sailed whereever you want to go, or so that's my impression when everyone kinda gets scattered throughout the land. And in the OT there were a lot of non Isrealites living in the land freely, working and doing business there.
It was a lot less complicated in those days, but there were foreigners in the promised land. Naturally there are conditions, but (don't quote me on this) so long as they abided by the rules, they were allowed to earn a living and contribute to the economy. Jerusalem was a trade hub right? Back in the day, pack your mule, you can enter any market your country is not hostile with.
Note that this is more of a work permit issue than an immigration/migration issue.
Of course this is just my oversimplification, in reality much more factors come into play, especially since the citizen, and indeed anyone on your nation's soil, has more rights and privileges than in the past where everyone was pretty much a slave to the king.
So as for my beliefs on this matter, I still feel oppressing the foreign worker is wrong and they should have some rights, such as fair wage, but taking in anyone is just not feasible, and it didn't happen in the past either. I would stop short of welcoming everyone in. Being a Roman citizen was supposedly great in the NT, a preveledge that was difficult to obtain. You can't even be whipped for breaking the law when in context law back in the day pretty much was whipping or execution for anything major. However if you wish to live or do business in the empire it was easier.
As for God's kingdom depends on how you look at it. No one is restricted from birth, everyone has a right to citizenship - conversion. Basically drop your old citizenship (whatever idols they were worshiping, or in case of atheists, overt materialism) and accept Christ. Whatever else you have to do else is debatable but I'm of the belief you're saved after you accept Christ. Other's definition of discipleship may differ though, and depending on who you ask, discipleship may not = default 'citizenship' of being saved, but something else.
Like I said, I may be wrong, this is what I understand so far.