From NSIS Wiki
Feature requests can be posted in the feature request tracker.
Keep in mind that there are a lot of commands that could have been added, but most of them will just make NSIS huge, and no one wants that. You will be surprised what you can do with the NSIS scripting language.
If something can not be done using the script language and you think it will be a useful feature, please give a detailed description of the feature you are requesting and make sure you don't post double requests.
If you have added a feature yourself, you can post the source code in the patch tracker.