To add ancestors, you just need to click on the person you want to add ancestors. On the left side of your screen, click on the button that says "Add parents" which will add a new set of parents to this person - this can be repeated ad infinitum.
If your question isn't about direct ancestors (aka a parent generation) but something like "My family descends from XX though I don't know every member from today to XX" - I don't believe this is possible. A workaround would be to add parents to the last known generation before XX, delete one parent, grey the other one out by using the gender option "other" and write something like "[Multiple generations of Y family]" and then add XX (and their potential partner) as parents.
Hope this helps!
