There was a data version of the minidisc. Sony tried a few times but it just never caught on. MD Data, Hi-MD were the two versions they came up with. They had devices like the PIT-IN designed to read/write data from the discs. I imagine if you search around online you might be able to find drives/discs but they won't be cheap.
Converting a pure audio unit to do data would be very challenging seeing the units were explicitly designed to prevent that kind of access.
I think if you just wanted some kind of cool looking, movie prop type storage I would hack apart a USB thumb drive and give it an interesting enclosure. MiniDisc never really delivered on its promise and are tricky to work with. I had several of the units and happily walked away when I moved onto MP3 players with hard drives in them.