Not sure what do you mean, in the agent based measures you can see Websphere Metrics (see attached pic). If those metrics are not covering your needs, you can also use the "Custom JMX" metric.
Also, in case that this could be helpfull, we have some plugins to retrieve metrics from Websphere:
I don't have right now to test this but try with this option, it should work:
1) Select Generic "Custom JMX" > "Custom JMX measure"
2) on the right menu " Browse Metrics"
3) Click on "Pick Agent"
4) Select the Websphere agent and the metric you are interested