Can I put other vegetables in a manual wheat grass juicer?
Some juicers are better than other kinds for different things.  A dedicated wheat grass juicers probably isn't going to juice spinach, carrots, and kale well.

A centrifugal juice usually doesn't handle wheatgrass well and in general isn't supposed to be as efficient as some other kinds for leafy greens.

A single-gear or double-gear masticating juicer works great for leafy greens, but isn't supposed to be as efficient for softer fruit.

If you're looking for something that will do wheatgrass, leafy greens, and a variety of other things - consider something like the Omega 8004.  Long warranty, easy to clean, and gives good quality juice.  If you need to make juice for a lot of people though, it is kind of slow for large quantities.