Yes you can register a read only repository that already exist, pretty common way to snap and restore to a new cluster.
Make sure you look close a the docs / correct version and version compatibility matrix.
If you register the same snapshot repository with multiple clusters, only one cluster should have write access to the repository. On other clusters, register the repository as read-only.
This prevents multiple clusters from writing to the repository at the same time and corrupting the repository’s contents. It also prevents Elasticsearch from caching the repository’s contents, which means that changes made by other clusters will become visible straight away.