Expander modules (digital ranks in a box) such as the Ahlborn Archive series (photo) have in the past been added to and interfaced with an existing electronic or pipe organ to augment the number of voices available.
There aren't many of these Archive modules in circulation, but you might still run across an instrument some day that has one.
If you're looking for one to add to your practice instrument, there's precious little to find these days because the market for them has shrunken.
If you don't mind used, and if you happen to get lucky, you might find an Archive module someone is willing to part with, they worked very well for the purpose but haven't been manufactured since around 2007.
What basically killed the module market is the virtual pipe organ [See blog, Virtual Pipe Organ (VPO)].
Mouse clicking is no longer needed for these VPOs; these programs and sample sets now work with a touch sensitive screen.
If you look into the Hauptwerk VPO, for example, there are oodles of sample sets to choose from now, some of which are really excellent.
You might also investigate jOrgan and GrandOrgue; these free computer programs are capable of delivering excellent sounds, and, if you're cash strapped, they may be just what you need.
The cost is just the downloads of the software and the time to get things up and running, assuming you have a MIDI to Computer cable.
If you want more control over the samples and have a budget, something like Hauptwerk might instead be a better way to go.
To buy the basic or advanced version of Hauptwerk isn't cheap, and a decent sound library adds to that, plus you need quite a heavy spec PC to make it work.
But what you will hear will convince your ears, very, very much.
Allen and Rodgers both have MIDI organ expanders with many individual stops as well as ensemble or chorus sounds.
New modules also can be had from Viscount and Content.
The Viscount is impressive and very configurable, but only puts out 12 stops at a time, and it's rather on the expensive side.
It perhaps works best as an augmentation to an existing MIDI equipped organ.
The Content module has more stops and could be worth checking out; the 220 model has some pretty decent sounds, not a lot of stops, but the basic necessities for a practice organ.
Artisan Instruments has a web site that can be checked as well, in case you're thinking of setting up your own VPO from scratch or converting an old analog electronic organ console to VPO using digital ranks.
(con't in Part II)