I use a range of online family tree apps (genesreunited and others) and often find distant family members who send me sections of their trees in Gedcom format, however I can't see any straightforward way to import a Gedcom into an already existing family tree either online or offline so what I end up doing is transcribing the data by hand.
Is there an accepted way to do this, or is there a tool that does it?
I generally prefer to avoid a straight import of other people's data like this because unless you're going to review it in detail first you are basically taking the accuracy of that research on trust, and if you are going to review it then it's probably just as easy to enter stuff by hand as you confirm it.
That said, most programs can probably do this - certainly I know that gramps can import files in various formats (including Gedcom) into an existing tree. Once the import is done you would then need to find people in the imported data that matched people who were already in your database and merge them.
Importing other people's data into your own is almost always a very bad idea. Too often, people are merging other people's GEDCOMs into their own, trusting that other people did theirs correctly and proliferating what are called "junk genealogies".
Not only that, you WANT to keep your work separate. If relatives send something to you, why go through days or weeks of work to merge theirs into yours, only to have them send you an updated GEDCOM six months later. That time could be much better expended doing your own research.
With regards to online trees, I believe you should only upload your own research, and not other people's. If they want their information uploaded, they should do it, not you. If they don't have the technical capability and ask you to do it for them, then you become the "trustee" to their data, and you should not pick and choose which parts to put up. They should send you what they want put up. Then the issue of how to import sections of their tree becomes moot.
Most genealogical software tools that you install on your computer (as opposed to a web site) will allow you to export a part of a family tree. How you specify that part will depend on which software you have, but in general, you can save people by name, by family, etc.
Once you've exported a part of a family tree, you can merge it into a different tree using the same software.
