If you want to evaluate a set of rules in a .drl file in your process, you should use a rule task, where the ruleflow-group attribute of the rule task matches the ruleflow-group rule header attribute (so those rules will activated if the process reaches the rule task node.
If you want to pass in parameters from the process, you should probably use an on-entry script on the rule task to insert the relevant data inside the working memory right before executing the rule. If you want to get results from the rule evaluation, you might want to insert some object (which is a variable in your process) in the on-entry script, in the rule make sure you set the result on that object and then in the on-exit script retrieve that value so you can use if (for example store it in a variable).