You can import a GEDCOM file as a new family tree, but you can not import into an existing family tree.
It sounds like you want to merge two family trees. If there is only one person who overlaps in both trees, then you can merge them manually. Here's a previous post that explains how to do it. If you're not confortable doing that, there are programs that merge two GEDCOM files together. Here's an article from Family Tree Magazine that looks at a couple of them. It turns out that merging two family trees is a pretty common thing for people to want to do, and if you search on Google for "merging GEDCOM files", there are a lot of resources to look at.
