1) You are importing the entire field group and it completely overwrites the existing field group with the same group key.
2) Create a second field group on site A. Duplicated the field in the group you want to copy the fields from. Move the fields into the new field group you created. Export and import this field group. Move the fields from the imported group to the place you want them.
2 Option B) Export both field groups, open the JSON files and copy the field definitions to the group you want them in then import that group.
Unless the fields are complex is would be easier to just manually recreated the fields on site B.
