No, the MemberTree program on Ancestry.com has no ability to combine GEDCOM files (e.g., uploading a file and having the program add changes and what is not already in the tree).
Most desktop genealogy software programs have a way to do this. It can be time-consuming to do this kind of merge, especially if you have a lot of changes.
There is an exception that involves acquiring one of Ancestry.com's FamilyTreeMaker's latest versions (2012 or 2014) which are supposedly custom-built to synchronize between this software and Member Trees. Neither version is glitch-free. And depending on what program generated your GEDCOM file(s), the interface between the FTM program and your GEDCOM is not trouble-free either -- one problem being how FTM interprets source-citations in GEDCOM files.
Nonetheless, if you go with FTM you can download your existing tree as an FTM file, then you must upload this tree again from FTM (do give it a different name). The two will be linked. Then use the FTM program to first import GEDCOM to FTM then merge it with existing FTM. Then you can 'sync' the result with your existing tree. All of this will take a lot of determination and patience. Many experience problems with FTM and synchronizing, not least because Ancestry.com has had a lot of server and software problems lately. Also FTM is a real memory hog, so if you don't have a lot of hard drive space and more than 3GB RAM, the program can hang up and cause other problems.
If you want to explore what to expect from FTM, see Russ Worthington's bloghttp://ftmuser.blogspot.com/
And explore the very active message board for FamilyTreeMaker under the Genealogy Software topic: