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

You would have to follow the migration guide and skip the step of installing Appian (since Appian is installed). This is needed because the migration guide tells you what files need to be copied.

Just keep in mind that PROD contains real data and as a best practice you shouldn't be extracting sensitive data from PROD to a non-PROD environment that might be less controlled.

@Eduardo what about the 2nd scenario?

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.