Most homeowners want to eliminate box elder bugs and consider them to be pests.  They're not harmful to humans, but they can be pesky and sometimes gather in abundance.  They enter homes hoping for warmth, which will allow them to survive the winter.  You can try to keep them in the terrarium and see if they survive and then release them in the spring.   They do like the warmth and, when inactive, don't really need food until spring.  In the spring, their diet consists of  sap and juices from trees, leaves, and seeds.  If they become active, you could feed them these items. 

Good luck with your project.