Trying to put a ball of yarn on a single spool would be really difficult, and unrolling it to roll it back onto the spool would be inefficient. I need to give you some terminology before I proceed, on different ways of storing/using yarn.
A skein is what you see in the store - it looks like a ball of yarn, maybe an oval shape. It's actually wound so that you can knit straight from either the outside OR the inside strand of the skein. It's also possible (but very difficult and annoying) to knit from both strands at once. You'd have to stop a lot to deal with twisting, though. These are skeins:
http://ww1.prweb.com/prfiles/2010/11/10/2842534/YarnSkeins.jpg
A cake is a flattish circular lump of yarn that comes off of a ball winder. Again, you can knit from the inside or the outside. Many knitters prefer to knit from cakes if they have a winder, but the yarn in a cake is kept slightly tensioned so it's not a good long term storage method. Here's a cake of yarn:
http://upload.wikimedia.org/wikipedia/commons/1/1f/Yarn_cake.JPG
A hank is a long loop of yarn. Some yarn bought from small sellers will come in hanks, or if you have a swift (not a squirrel cage swift) you can create a hank from a skein or a cake. Here's a hank open:
http://upload.wikimedia.org/wikipedia/commons/d/d4/Opened_yarn_hank.JPG and closed:
http://static.flickr.com/34/94934367_7c4b83de49_b.jpg A hank is hard to knit from directly UNLESS you have a squirrel cage swift. You can knit from a hank using a regular swift (vertical or horizontal) but it takes up a lot of space. This is a swift:
http://c-cactusflower.com/mcart/images/yarn%20swift.jpg
If your wife does not normally buy or create hanks (or express interest in doing so), then she doesn't need a swift. If she wants to make hanks, a squirrel cage swift won't work, but a regular or vertical one will. If she wants to knit FROM hanks (as I often do), a squirrel cage swift is fantastic and compact, but will mean her knitting won't be portable until she finishes using the hank, as her project will literally be tied to it!
If your wife normally knits from skeins or cakes, you might try incorporating a couple of yarn bowls. They look kind of like this: www.etsy.com/listing/111224202/ceramic-heart-yarn-bowl but there are a lot of different styles. Basically it's a bowl with a little looping bit cut out of it to hook the strand of yarn through. The skein or cake might bounce around in the bowl a bit as it unrolls but will stay put.
When working with more than one yarn, some knitters prefer to have each strand coming from opposite sides to help not get tangled. So you might make one of the bowls removable so she has that option. Just make sure to give it a heavy base so it won't move around on its own when disconnected.
I hope this isn't too confusing! Let me know if you have any questions.