Scoring callout doesn't have a direct method to get UserId by design as its main purpose is to provide the Scoring capability. There can be a rule set which doesn't require UserId.
Having said that you can use the following method to retrieve any value ( i.e. in your case set it to UserId) that is set by EvalCallout.
scoringDoc.getScoringCallout().getCalloutModifier().getModifier();
Set this value in Eval Callout:
EvalCalloutResponse evalResp = EvalCalloutResponse.Factory.newInstance(xmlOpt);
evalResp.setModifier(<UserId>);