Hi  Guillaume Morrissette ,

For the new project creation can you please click  from project view  on Create new project. Then on the wizard that appears please  check if clicking on the add new files (like in the below image ) it allows you to add the files?

In regards to the  files you have translated in the past, if you still have the source document and the target ones you can use the alignament in Studio and create a translation memory that then you can add to your existing translation to give you segments translation suggestions. At the below 2 links you can find more details on how this tool works. 

https://www.youtube.com/watch?v=I9DGXVHkeYk 

https://www.sdltrados.com/solutions/translation-alignment/ 

In case the old documents you translated were translated in Studio then you should have the SDLxliffs that you can import into a translation memory- https://community.sdl.com/product-groups/translationproductivity/w/customer-experience/3354/how-to-create-a-translation-memory--after-you-have-finished-a-translation-without-using-an-enabled-tm 

If you are looking at having also a glossary in Studio then you would need to create a termbase to get also suggestions for single words. 

Do you use the trial version of Studio Trados or another version? 

Best regards, 

Loredana