If you're invoking your action within the transition away from your view-state, you can pass the request parameter in directly with EL:
Code:
<view-state id="whatever">
<transition on="someEvent" to="anotherState">
<evaluate expression="objectInScope.methodToInvoke(requestParameters.desiredRequestParameter)"/>
</transition>
</view-state>
If you're invoking the method in a separate action-state, you can either use the same EL to get the requestParameter, or you can store it in some other scope (flash, flow) in the transition of your view-state, depending on your scoping needs.