Pre-made seasoning to add to rice in rice cooker to make rice more flavorful?
You can combine it with practically everything, so the question is somewhat broad. So my answer is equally broad: rice pairs well with fresh tastes and acidity, or with moderately sweet components. Or you can just underline its own slightly nutty notes. Below is a list of specific examples, but it is impossible to make it exhaustive. 

For fresh tastes, use herbs. Summer savoury and spearmint are the classics, I never make stuffed peppers without them. But other light tasting herbs are also a good choice - lemon balm, oregano. Rosmarin is sometimes good, but somewhat overpowering, it is better for a dish where the rice is cooked together with veggies (eggplant, zucchini). 

Beside herbs, you can try more exotic seasonings. Lemon zests are good, finelly chopped grape leaves and shoots are great. 

Coriander powder should give you a similar taste profile to cilantro, but it is easier to keep at hand for when a quick dinner without much planning is needed. 

The sweet option is also interesting; if you don't overdo it, it is OK to serve it as a side dish for a savoury meal. The easiest way to achieve it is to cook the rice together chopped dry fruit. Sulfured raisins are popular, the dish will be somewhat si milar to pilaf. But others work too, I especially like dried apricots because they are slightly tart. 

Other methods for sweetening don't really fall into the "dry seasoing" category, but are  worth mentioning. You can add fruit juice to the cooking water (apple or other slightly sour juices are best; this will affect the starch in the rice, causing it to cook firmer than usual) or you can add a small amount of sweetener to the cooked rice (prefer aromatic sweeteners lik honey or C grade maple syrup). Or if you prefer it spicier, mix it with a sweet chutney, like mango. 

If you want a subtler taste, combine the rice with nuts (cook them together). Always use nuts with the brown skin removed. I have found unroasted, finely chopped nuts to work better this way. Hazelnuts and almonds are a very good choice. Para nuts also pair well with rice, but are seldom availablr blanched+chopped. This works especially well if you use the parfrying method for the rice, frying in the oil of the nut used. However, I don't know if you can parfry rice in a rice cooker, I always make mine on the stove.