Ability to override automatic tax calculation in OM

Our OM users sometime want to be able to override underlying calculated tax.

In OM however the tax window is not updatable as it is in AP and AR. Is there a way to do it. Can we use 'tax classification codes' to achieve this. e.g. if we have a tax calculated GST5%  and PST7% but our user want to override it  to HST13%.

I have used tax classification codes to exempt tax codes, but not to override tax code A with tax code B. Has anyone achieved this. Would really appreciate if you could share your experience with this.