Within DPA, click on "Options" at the top right of the UI.
On the "Monitor Setup" tab, look to the right and you should see a "Custom Metrics" button - click on that.
Click "Create New Custom Metric".
Fill in the fields appropriately. (see image below)
Adjust the value for the warning or critical level for when you want to be notified (or leave unchecked if you just want to start plotting signal wait time as a raw metric.
You will want to use a delta since the counter we're querying is cumulative.
Also, if you have a different query you use to collect this data, feel free to use one to your liking!
SELECT SUM(signal_wait_time_ms) AS [SignalWaitTime]
FROM sys.dm_os_wait_stats
