Sibling with father not married to mother

In my tree I have a half brother who we thought had the same father as my other half brother and one of my half sisters. It turns out that his father was actually our step grandfather (our grandmother's second husband). Obviously this is not a relationship that appears in the drop down when choosing parents under the Relationship tab. This person (the step grandfather) appears on the tree, but as a husband of our grandmother and a step grandfather to all of us. How can I select him as the father of my half brother along with our mother as his mother? Is this even possible?