(sorry for bad english, it's just a quick and dirty translation - I'll update it later)

You can use add custom performance counter to agent's config.

Configuration sample, add this to nxagentd.conf:

*WinPerf
Counter = DiskWriteBytes:"\PhysicalDisk(_Total)\Disk Write Bytes/sec":60:A:INT:Average number of bytes per second written to disk for last minute

"*WinPerf" - section name
"Counter" - counter definition. Format:
    Counter = ParameterNameForNetXms:"\PDH\Counter\Name":SamplesCount:Queue:Type:Description

"ParameterNameForNetXms" - this name you will see in management console, in DCI configuration
"\PDH\Counter\Name - Counter name, e.g. "Memory\Available Bytes".
"SamplesCount" - number of samples taken for calculation of average value
"Queue" - querying queue, it can be: A, B or C. A - counter will be read every second, B - every 5 seconds, C - every 30 seconds.

Example: if you will set SamplesCount to 60 and queue to A, this parameter will return average value for last minute. count=30 and queue=C - this will be average for 15min.