How to access Bucket from another account to FSx

Here's the documentation for linking an encrypted S3 bucket in a different account: See 'Accessing server-side encrypted Amazon S3 buckets in a different AWS account' section here: https://docs.aws.amazon.com/fsx/latest/LustreGuide/create-dra-linked-data-repo.html#s3-server-side-encryption-support

How to add data association in FSx if S3 bucket is from different account?

Refer the AWS blog https://aws.amazon.com/blogs/storage/protecting-your-high-performance-file-systems-with-amazon-fsx-for-lustre/ to configure your filesystem with cross account s3 bucket.