Hi, 

You can still plant it. It is better to have it in the ground over the winter (if you already have it) rather than remaining in a container. Technically, you can plant anytime the ground isn't frozen, and since the plant will be dormant soon, it will transfer easily. Just continue to monitor for watering needs by feeling the soil and supplementing water whenever it is dry. If we have some warmer periods over the winter and it hasn't been raining or snowing, check on the soil and see if it could use water. Using a biodegradable mulch like leaves or wood chips will help to insulate the roots; just don't pack it up against the trunk or branches. If you have deer that visit your yard, you will want to fence off the tree to make sure they don't sample it over the winter. Usually, Spicebush isn't a favorite due to the strong-smelling branches, but hungry deer will try anything. 

Let us know if you have further questions. 

Emily

Some helpful resources:

Planting a Tree or Shrub

Mulching Trees and Shrubs

Watering Trees and Shrubs