(There are many forum threads about this already.)
You can convert the key to the required format using PuttyGen.exe, one of the companion tools on the Putty website.
It's possible the server will still reject the key if it's configured to not allow any of the encryption algorithms Opus supports, which we don't have a workaround for at the moment. But not all servers are like that.
We're in the process of replacing our SFTP code with a new library but don't have an ETA yet. It's being worked on right now, having finally found a library which can transfer with good speeds over fast networks.