Add a post-function in your workflow's "Create" event so that it can do whatever you want, like changing the assignee to someone based on your conditions above. Take a look a the JIRA Misc Workflow Plugin and the JIRA ScriptRunner to accomplish this, they have tons of information and samples in their documentation.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.