Can i use Transforms to collapse data?

Elastic Stack Elasticsearch
5

If i use tranforms for aggregate documents, i cant save not used fields in group_by to dest index.
PUT _data_frame/transforms/testconc
{
"source": {
"index": "from"
},
"pivot": {
"group_by": {
"period": {
"terms": {
"field": "period"
}
},
"field_a": {
"terms": {
"field": "my_field_a",
}
}
},
"aggregations": {
"u": {
"sum": {
"field": "u"
}
}
}
},
"dest": {
"index": "trindex"
},
"frequency": "5m",
"sync": {
"time": {
"field": "createdAt",
"delay": "30s"
}
}
}

But document have my_field_b (can not exists) and i want save it to destination index.
my_field_b from any document from grouped bucket.
for example, the existence of aggregation "first"/"any" would help me