Is it possible to determine which users / group / role will

In the documentation I found the following Server-Side API:

 rbv_api.startApproval(objName, objId, actionId)

This allows me to start an approval process at the moment that I want it to start which is great.
But I am missing the possibility to select the users for the Approval start. Yes I can define those in the object definition, but in real life approvals are dynamic most of the time and not a predefined list of users. 
So actually what I want to achieve is that the creator of a certain item to be approved selects the users who will need to approve that item. Then I want to start the approval process using the selected users (and not the predefined users in workflow action). Or give me an api to influence the users for a certain workflow action (for a certain object record of course). 

Is something like this possible already somehow?