we have been using Socialcast at work, but since it will be shut down we are looking at Yammer for an alternative.
As far as I know Yammer does not even support importing the information that was exported from Yammer.
Is there ANY way to migrate the data from Socialcast to Yammer?
There isn't a good way. You could write a script that impersonates each Yammer user in order to post the old messages from Socialcast. However, the timestamp of each post will be the timestamp of when the script ran rather that it's original timestamp. If you're ok with that and with writing the script to post all the messages, replies, and attachments, then this could be a viable option.