3d modelling from existing stl's (combining various characters and shapes)

Looking to 3d print a fantasy scene diorama, which requires some 3d modelling (CAD) to be done on top of existing stl's - I have a character's face stl, it would need to be transposed onto another body. Would suit someone with mastery of blender - final mesh needs to be clean and converted to stl to be able to be printed. 3 figurines in total.