You can use the "Blended Family" option, which can be flipped on and off, at the Family Tab of the People Workspace.
FTM does provide for adopted or step children, using the steps that Curt has provided.
However, I suggest that you reconsider doing that.
You don't mention if this is a current day person or a prior century ancestor, which might make a difference in your mind.
The caveat here is that if you make a person a step or adopted child of a person, they, and ALL of their descendants will appear as descendants of that person in descendant charts & reports. Further, they will appear MORE THAN once in some descendant charts & reports; once under each marriage, like the Outline Descendant Report. In a Genealogy (Register) Report, if the "main" parent is selected as the "selected" person, the step or adopted child will appear twice with the same descendant number under each parent, and the descendants of that step-child will appear later in that report only once. So, that is good, but the report won't say which parent is natural and which is adopted and why the step-child is in the report twice.
In short, you end up with a mess once you start putting non-blood relationships into FTM.
I am a strong believer that genealogy should follow blood-lines, so I don't attach EITHER step OR adopted persons as a child of the step or adoptive parent.
This can also effect ancestor reports depending on who you have selected as preferred parents. Preferred parents selected at the "Relationship" view of the Person tab will determine which side is chosen as ancestors. You can end up with non-blood ancestors showing for a person if any step or adoptive parent up the tree is a preferred parent.