PEMapModder once wrote a gist that executes this transfer, but it did not work properly and needed fixing. Search through his gists to find it, or search #pocketmine IRC on botbot; there was a link to that gist on IRC too. I would however recommend you to use alternatives. For wxamp, HereAuth is much better than SimpleAuth in terms of performance especially with mysql data provider, and it has an /import command for importing database from SimpleAuth (you have to provide details).
Just as a sidenote: http://stackoverflow.com/questions/28805561/how-to-import-yml-file-into-mysql-database
Note that this is not useful as SimpleAuth does not have the same structure in YAML and in MySQL. Manual translation would be required.
If you want to convert SimpleAuth yaml to MySQL (and then to HereAuth.... ), this should do the trick - you'll just have to get Spyc and replace the correct paths: ps I just dug this out of an old backup, so please test it before you run it on a production system
Indeed, and I'm not even sure Spyc still exists! Might help someone get it done though, and it's worth doing if you've got a lot of registered players
If you mean the original is also MySQL, just dump the database and import it on the new server. If the data is in yaml files, you can use the script above to import the data into a MySQL database, then dump and migrate.