Every ex slave that served in Mr. Lincoln’s Army enlisted into that army. The were not “pressed” into the army.
“The reason many ex slaves escaped into Canada” was because of the Fugitive Slave Act and the Dred Scott Decision. Neither of which could be enforced in Canada.
I don’t know what to tell you, you’ve either got some excellent rose coloured glasses on, or you don’t know history.
Too many Hollywood movies then?