Just adding a minus sign won't work, as any characters other than numbers will not be recognised by the tree. I don't think there is any way to input BCE dates directly. What you could try is putting a blank box as the 'parent' of the last ACE person, then putting something like: 'All dates from here are BCE' as the name. Then put the real parents of the last ACE person from that box, with their dates in BCE format, such as: '5' in the birth date if they were born in 5 BCE. This isn't ideal obviously but it is workable.
By the way, well done on tracking a family back so far!
