That's very easy. This is why we programmed Marvikshop.
The tables are the same! The only different is the name of the tables.
They called for example mcyuk_OSC_PRODUCTS instead of PRODUCTS
mcyuk = joomla
OSC = the component Marvikshop.
The best way to migrate is export and import tables. You only have to add mcyuk_OSC to your tables. And sometimes you have to add some extra colums because of the installed contributions from oscommerce.