1. Is it possible to migrate process instances from one environment(Like PROD) t

That's not possible by itself but you have different ways to achieve this:

1. Extract the information from the legacy system using any of the integration capabilities of Appian such as Web Services, Custom Smart Services, REST, Querying the Database, etc

2. And you can then decide whether the extraction is read-only to display the data in Appian or if you want to read and create records/processes/etc in Appian to "migrate" the data.

That's not possible by itself but you have different ways to achieve this:

1. Extract the information from the legacy system using any of the integration capabilities of Appian such as Web Services, Custom Smart Services, REST, Querying the Database, etc

2. And you can then decide whether the extraction is read-only to display the data in Appian or if you want to read and create records/processes/etc in Appian to "migrate" the data.