Hugh, in 1860 20% 0f Illinois residence were foriegn born.In the metro areas of St.Louis and Chicago as high as fifty percent.I would assume higher (then 20%) in PA.No need to back up the troop trains to the boats they already were here and were agressively sought after(recruited)probally for reasons you stated.I found this interesting.
http://www.lib.niu.edu/ipo/1997/iht429702cm.html