Supporting humans will require removal of CO2 they produce, not addition of CO2 from external sources. You will need it for agriculture, but the bulk of it can come from recycling organic waste. People will be living on imported food until agricultural production is capable of fully replacing it, so there will be plenty of that.
Nitrogen will pretty much have to be imported. The moon is also hydrogen-poor, so perhaps import most of it in the form of ammonia for agricultural and industrial uses. Any excess hydrogen you end up with can be combined with oxygen from regolith to make water.
The LCROSS impacts revealed the presence of methane, ammonia, and carbon monoxide and dioxide in a polar crater, so at least some of it could in theory be sourced from the moon. This will require mining ice in an eternally-dark polar crater, however, which will require mining, road-building, lots of machinery, etc, not to mention study to determine exactly what ices are available where and in what quantities.