3DNA does not provide an automatic way to build a dsRNA with non-Watson-Crick base pairs, e.g., the G-U wobble pair. Nevertheless, it has the components that can could possibly be combined to get the job done, on a case-by-case basis. If you are specific about what you want to achieve, our discussions can be more concerete. Alternatively, you may try software tools developed from the laboratories of Eric Westhof or François Major, among other possible choices.
Xiang-Jun