I've uploaded another version, you can find it on the first page of this thread.
Now you can merge multiple SEC files into one.
In addition, under Settings > Export Options, you can adjust the coordinates of a SEC file. So you could move the whole mesh 2000 pixels to the right or 180 pixels up. Usually you won't need that option but it might come in handy if you want to merge existing maps into a single big one. It's a beta feature because I haven't really tested how it affects ramps and stairs.
A great sec making tool. 3000, what is your email address? I have some allied death squads 1. I would like to ask you for advice.