Yes, you will have to export multiple GEDCOM files and combine them. You can export a GEDCOM from any profile you have added, so it should be possible to export all of those. For profiles already on Geni you may have a problem, but if you add profile near them those should also be included in a GEDCOM.
It will also depend on which profiles you wish to include (if your profile is connected to the Geni World Tree you are connected to more than 100.000.000 profiles). You can make an ancestor report from your profile to see your direct ancestors.
There are some utilities that can combine GEDCOM files or add a GEDCOM to an existing database. You will have to search to see if you can find (a free) one that works good enough.
You can export an activity report (https://www.geni.com/account_settings/download_activity) to find the profiles you have added.
It is a lot of work!
If you are a programmer you might be able to use the Geni API (see https://www.geni.com/platform/developer/help/api) to write a program to make an export, but that also will be a lot of work.
I tried to foolishly upload my very large tree on Geni to Family Tree DNA. Unfortunately it only put three people on there and wiped out my existing tree on family Tree DNA - something I spent two hour creating!
You need to put HUGE WARNINGS on your site about this!
Transferring an existing tree to someplace else is Not easy and there is ZERO warning about what to do and how to do it or what not to do!!!!!!!
Peter, there is a limit on how many profiles you can export from Geni. This is explained on the GEDCOM export page.
GEDCOM exports for Basic users are limited to 4x the number of profiles you've added and up to a maximum of 5,000 profiles. We see that you have only added 1 profile to your tree. Therefore, your export only contained 4 people.
See our FAQ: https://help.geni.com/hc/en-us/articles/229705167-How-can-I-export-my-GEDCOM-
You will have to contact Family Tree DNA directly to see if they can restore your tree on FTDNA.
