With a command button :
PHP Code:
<p:commandButton action="myAction">
<f:param name="myParam" value="myValue" />
</p:commandButton>
With a command link :
PHP Code:
<p:commandLink action="myAction">
<f:param name="myParam" value="myValue" />
</p:commandButton>
you can then access those parameters in the flow definition file through the requestParameters variable for example:
PHP Code:
<subflow-state id="myAction" subflow="mySubflow">
<input name="myParam" value="requestParameters.myParam" />
</subflow-state>