The way I have worked by linking these data is:
CRMD_CMG_CASE_AT-CASE_GUID = SCMG_T_CASE_ATTR-CASE_GUID
then
SCMG_T_CASE_ATTR-EXT_KEY = CRMD_ACTIVITY_H-EXTERN_ACT_ID (be careful because EXTERN_ACT_ID is char(20), so you need complete with 0 left zeros the empty positions, for ie: ext_key 2032 is equal to 000000002032 EXTERN_ACT_ID)
then
CRMD_ACTIVITY_H-GUID = CRMD_ORDERADM_H-GUID
I hope it'll be useful. Regards
Nati