That information comes from 3 extended properties that are set on the message eg
So you can use a patch to update those properties yourself eg
PATCH https://graph.microsoft.com/v1.0/users('gscales@datarumble.com')/Messages/AAb7CR....
User-Agent: GraphBasicsPs101
Content-Type: Application/json
Host: graph.microsoft.com
Content-Length: 922
{
"singleValueExtendedProperties": [
{
"id": "String {41F28F13-83F4-4114-A584-EEDB5A6B0BFF} Name ApprovalDecisionMaker",
"value": "Dave Dobbin"
},
{
"id": "SystemTime {41F28F13-83F4-4114-A584-EEDB5A6B0BFF} Name ApprovalDecisionTime",
"value": "2022-03-03T10:52:57.3068905+11:00"
},
{
"id": "Integer {41F28F13-83F4-4114-A584-EEDB5A6B0BFF} Name ApprovalDecision",
"value": "1"
}
]
}