Arbortext themselves have a tool to achieve that, it's called Arbortext Import/Export and it can take MIF files as input.
Are you using unstructured FM? If so, the conversion is to a structured format (XML) so that will mean quite a lot of work in setting up the conversion maptemplate and also lots of QA for each file. Some organisations choose to use an offshore conversion vendor if they have a large enough volume of conversion work.
You will need an import/export license.
If you have only a handful of documents and/or only a couple thousand pages, consider using cheap labor to do a copy/paste conversion.
A little on topic, a little off topic, slightly rhetorical - why do these software companies make it so difficult? Isn't the utility of using XML that it can (should) be easily shared between users (software/ hardware/ vendors/ suppliers/ customers...). I guess XML is universal, but the software code to make it pretty is not. ugh
Like any technology I think it is evolving. I'm pretty sure the first cars weren't cheap, reliable and easy to operate...