Configure a threshold alarm on sum of data sources

[mgpm2]

I'm monitoring a device which exposes CPU stats over SNMP. It exposes three separate metrics: CPU_nice, CPU_system, CPU_user. I've configured all three of these as separate data sources. I then have a CPU usage graph which draws lines for each of these, plus a "Total" line which used a CDEF.

Now I want to configure a threshold alarm for when Total CPU usage exceeds a certain percentage.

When I configure a new threshold, I have to first select a host, a graph and a data source. So I am forced to select one of CPU_nice, CPU_system or CPU_user before I can configure any other properties of the threshold. In the threshold definition I can configure an RPN expression which can reference data sources, so I thought I might have been able to do "[ds:cpu_nice],[ds:cpu_user],[ds:cpu_system],+,+" but this doesn't work -- it seems that RPN expressions can only reference the data source that the threshold is being configured for.

Is there an alternative way to achieve what I need here?