This is where I have to try to be diplomatic and I am not very good at it.....
There is no need to actually name a parent of a child in your family tree, though if that is what you want others will advise you of how to do it.
A couple of times I have found that a child in my family tree, we'll call this one Mary, is not ( or maybe is not) the daughter of her mother's husband (usually it is because the child was born before the wedding..)
So I have given her a separate father and called him Mary's Dad, because it sounds nicer than Unknown.
I'm sorry for misleading you. Online family editor doesn't let you add this type of relationship indeed.
You can use our Family Tree Builder software to build your tree, there you'll be able to add your father as your partner. You can download Family Tree Builder by clicking Family Tree Builder link at the bottom of the page under Downloads section.
Once you install it and add all the information and names, click 'Edit' on your personal card, then select Spouses > Attach spouse option to connect your father with you as partner.