Amazon RDS now supports cross account KMS keys for exporting RDS Snapshots

Amazon Relational Database Service (Amazon RDS) now offers the ability to specify an AWS Key Management Service (KMS) customer master key (CMK) from a different account when exporting an Amazon RDS Snapshot to Amazon S3. This option helps customers organize and consolidate their KMS keys by eliminating the need to create keys in each account that has snapshots.

Source:: Amazon AWS