Such a force can be managed by UNSC or the secretary general. It can include rotation spec ops units from participating countries. If a country objects to the mission, than its troops simply don't participate. Should be exclusively a counter terrorism force, in cases where there is a credible international threat. Should be much smaller than Nation Armies as to pose less of a threat. Under this system, it can be done.
One of the issues of the UN is that they are not always recognized as powerful. With a standing army it would make them have legitimate power. Perhaps dividing the army participation equally among countries in the UN would be the best way to accomplish this task of an army.
No UN army would ever be a truly UN army. A handful of countries supply the vast majority of funding for the UN, and almost every UN action has been undertaken largely by a handful countries (mostly Western countries). Having a UN army would just add to the mess that is the UN.