Thank you for your question. I am not the most knowledgeable indvidual on this forum I would say Kirby, Nichols and Mr. Martin among a few others are much better informed on this subject then I. I have a BA in history but I didn't study the CW much back then. I seem to recall in Southern Boy in Blue Marcus Woodcocks memoirs of the 9th Ky Vol USV that new recruits did come into the regiment from time to time. One factor that would make it it difficult to accept new recruits from say Mo was that most Mo USV regiments (see civilwarregiment.com) often served far from home and it would be difficult to find said regiment to join it but not impossible. Could your character join say a controverisal unit such has the 2nd Colo or 7th Kan? Has you know union regiments recruited local southern men I would not be surprised if visiting USV's in MO recruited local men. There is a 2nd Colo renactment society in Mo that would be an interesting question. On the other hand there was a high attrition rate in all regiments North or South from disease, death, injuries and desertions so there was always room for eager young and sometimes not so young men and even women in disguise.

I hope this helps a little bit,
Leonard