This may already be possible, but I would like to have a email alert when a server reaches less than 10 % of free disk space. Is this possible?
This is definitely possible, and for Windows2008 it's already enabled "out of the box" (using a what NMIS calls a 'Threshold Alert').
Have a look at the documentation for Basic and Advanced Thresholds in NMIS8, which describes this component of NMIS; if your server is a Windows2008 system you'll find the relevant threshold settings in its model under alerts – storage – HighDiskUsage. This alert's trigger definition should also work for most other models where disk storage is being collected.