Here you are my two cents from migrating emvideo fields. This patch is ready to be extended to support more emfield modules like emimage, emaudio, ...
Instructions for developers:
The example field mapping code below shows how to import video urls in emvideo field.
Note that these code snippet must go into your Migration subclass's constructor - see the migrate_example module for an example of how and where to use field mappings.
// Import source field "url_video" containing urls into
// destination field field_emvideo:
// Import source field "url_video" containing names into
// destination field field_emvideo (youtube provider is the default,
// so it's not needed to pass this argument in this case):
$arguments = MigrateEmvideoFieldHandler::arguments('youtube');