Hi Marcos,
I took a look through our other JIRAs and asked some of our developers about it, and it seems that both .COPY and .T3D are legacy UE3 options. We don’t have a specific plan to remove them, yet, but the expectation is that Blueprints shouldn’t be able to be exported.
There are a couple of other options for moving Blueprints that may help, however. First, the Migrate feature will allow you to simply move the Blueprint and any relevant assets from one project to another. If you don’t have access to both projects on the same machine, you could instead copy the graphs in a Blueprint and paste it into a text document. This becomes a little more complex once you involve multiple functions, as the Blueprint and each function needs to be created in the receiving project before you can copy the graphs over, but it’s an option.
Additionally, .uassets can be moved to and opened in a separate project directly as long as all references are moved with it (with the same file structure). This is basically what the Migrate feature does.
Hope that helps! Right now, there’s no easy way to use T3D files in the manner you suggested.