Allow Custom Entry In 'Key' Field

I'm pretty sure that this would be a small modification.
I would like to be able to enter a custom, or user defined entry in the 'key' field.
I use 'Mixed In Key' to scan my tracks and that puts the keycode in that field, for instance 1A, 2A, 3A etc. However there are a few tracks that it didn't identify, and I am unable to enter the information manually as there is no option in the drop down list.
I would like to be able to enter a custom, or user defined entry in the 'key' field.
I use 'Mixed In Key' to scan my tracks and that puts the keycode in that field, for instance 1A, 2A, 3A etc. However there are a few tracks that it didn't identify, and I am unable to enter the information manually as there is no option in the drop down list.