I dont think users can create new declare expressions and decision rules during the runtime. However, end user can make use of the rule delegation feature for modifying.

Rule delegation allows the business line to configure simple application logic without involvement from IT. For example, you can delegate a decision table to a business line manager. The line manager views the decision logic in a familiar environment, such as the Case Manager portal, and can update it as a stand-alone item without knowing all of the related, technical details.

Can you please provide more details on the usecase where the end user is required to create the declare expressions and decision rules?