If you're using the WordPress Plugin embed method, you can import your existing WordPress comments. This can be done at any time from the ReplyBox dashboard.
If you haven't already, sign in to ReplyBox. Then head over to your site dashboard and navigate to the Embed tab. Click Import Now.
The import can be ran multiple times without issues. Only comments which don't already exist in ReplyBox will be imported. Comments which have previously been imported will be skipped.