Time in the bottle..though if you are looking to increase maltiness, and body, you can add adjunct malts such as carapils/Dextrine, and other more "specialty" malts that end up with more non-fermentables. If you were doing all grain I would say raise the temperature of your steep as well, hit more into Beta-Amylase than alpha.
I recently did a Belgian Golden Strong, and it finished at 1.005. Which is completely acceptable for that type of style and in my case desirable for my end product, which has a wonderful taste and is terribly refreshing and dangerous due to its 10.2 percent ABV content. The kit you got may not have really been aiming for a full bodied trappist ale. Trappist is more about place of origin than style, many of them are somewhat similar, but there are quite a few differences between them. I believe and I'm sure someone will correct me if I am wrong that there are technically only 7 trappist abbeys, and they have a trademark right to the title, though some do "lease" their names to outside breweries, many such as Orval employ full time brewmasters that are laymen.