I was curious if the prohibition of entering a church extends to going to areas of the church that are not used as the sanctuary for 12 step program meetings. These meetings are almost exclusively held in church basements and are very important for people dealing with addiction.
If the meeting is (as you write) very important for dealing with the addiction, and there is no suitable course outside of a church basement, it is permitted to enter. The principle prohibition of entering applies to the sanctuary alone.
Although authorities are generally careful to ensure a person keeps his distance from a church, many have permitted entering the basement for the purpose of voting. There are special reasons for this leniency, and in particular the fact that the use for voting is well known. However, this will apply to some degree here, too.
Please see here for more sources on the matter.