I believe that other then adopted or foster children, step children can't be directly added to the tree, but you can turn a child into a stepchild/add a person as step parent. There are two ways to achieve this, but for both, you'll need to create bio parents and child by normal means and add the designated step parent(s) to the tree as well.
1) This solution is faster, but only possible when the step parent is a partner of a bio parent (which is probably most cases, honestly). You click on the child, and select the button "Add second parents" on the left side of the screen.
This opens a big bar of options. One of the lower options should be "Set stepmother/-father to [bio parent's new partner]". If you select this, the tree will recognize the new partner as step parent.
2) Here too, click on the child. Then select "More Actions" on the left side of your screen. The first option to show up should be "Set Parents".
If you click on it, you'll see options for Primary, Second and Third parents. If your kid is supposed to have a step father, you'll click on the field for Second parents - father and select the designated step father. Below this, you'll get the option to select "Type of second parents" which allows you to specify whether the parent is biological, adopted, foster, step, surrogate or someting else. Beware: If a child is supposed to have two step parents because both bio parents have remarried/new partners, you'll need to add the two step parents as Second parent and Third parent, otherwise the family tree will treat both step parents as if they were a couple.
Two last notes: You'll need to set the step relationship for every kid manually (if you add a sibling to a child with a step parent, it won't show a relationship to the step parent). And it is possible for a child to only have step parents and no bio parents if this is what you're looking for: Just add a child and his designated foster parents, go to "More Actions" and "Set Parents" again and select the Primary parents' type as "Step".
