I think I understand the question... you basically want to put the guts of the laptop into a bigger box, and then will have room for a bigger drive.
I know you can get adaptors that let you plug a 2.5" drive into a desktop, but I have never seen an adaptor that would let you plug a 3.5" drive onto a laptop motherboard. Of course this doesn't mean it doesn't exist, but I suspect it would be a lot rarer since you just aren't going to see a lot of use for it.
You may have to fashion one up for yourself. If you can get your hands on the appropriate connectors, and if you're handy with the soldering iron, you could hack one up for yourself.
The only other issue is going to be power; 2.5" drives only need 5v so it's easy to pop them into a desktop system, but 3.5" drives (at least every one I've seen) needs both 5v and 12v. You'll have 5v from the laptop mobo but you'll need a normal power supply to provide the 12v. (Maybe your box you mentioned has a normal power supply, I'm not familiar with it).
Anyhow, from just an electronic signal point of view, 2.5 and 3.5 are both IDE, it's just a different physical connector so an adaptor is certainly feasable.