Suzanne, this is not a tree so it does not have a trunk. I've talked to several folks who specialize in house plants and most suggest you use the technique called air layering to shorten the stem. Planting the stem or as you call it, the trunk, so deep in the soil, you risk having it rot. Google "how to air layer" for tips. You can plant the air roots if you wish. They will stabilize the plant. Most folks cut them off. Hope this helps. Happy Gardening, Nancy.
Piling up soil in the stems will cause them to rot. Best And Happy Yardening, Nancy.