Elastic SIEM is "only" a Kibana feature. It's fully built on the assumption that your underlying data is structured in our ECS format.
While we have an alias datatype this won't fix any data types. IMO your best bet will be to ingest your data and change it to be in the ECS format (potentially with a Painless script in the ingestion pipeline or alternatively with Logstash if you're using that for moving data around).