Yes...they can. The reason they call it a kit is because they match the timings and characteristic of 2 sticks of ram, it isn't made special or anything. Theoretically, you could buy 4 sticks from 4 manufactures and put them all in and they'd work. However this sometimes can cause problems as manufactures specs are sometimes off a little.
As long as you buy the same model ram, you can always add more later and be pretty safe though.