I have seem performance counters like this:
PhysicalDisk(_Total)\Avg. Disk Read Queue Length
Which is an aggregation (average, sum, etc) of data over a period.
How to create a custom counter which is an aggregation of existent data of another counter?
I don't think what you want is built into perfmon. You could always take any counter and export it to Excel. There you can do any sort of data manipulation that you wish.
Check this product: http://www.wextwise.com/kashef.html
You can configure collection from multiple performance counters than define aggregation rules using Linq.Net. there is a free version also.