Adding bones to two meshes

Hi all, I am a beginner to Blender and I am encountering some difficulties regarding adding bones to a mesh. I have two meshes (a body including arms and legs and a head. head and body are separate). I made a mistake where I had forgotten to join the two meshes together and I am now finding it difficult to add the bones and make it look good. (when I try to join them together, the hair on the head goes weird and the head shape changes).

Is there any way i can recover my mistake without having to start again?

Thank you for any help that you can provide.