Welcome to the ODK forum, @Felix_David_Hernande ! We're glad you're here. When you get a chance, please introduce yourself on this forum thread. I'd also encourage you to add a picture as your avatar because it helps build community!
It is possible to use Briefcase to migrate data from another ODK-compatible server to Central (for example, from Aggregate - see How to migrate data from Aggregate to Central - #2 by LN for a brief discussion about it), but that method will not work to move data from Google Sheets to Central since a Google Sheet is not a really a server.
You might be able to do something with Central's fully featured REST API (possibly using pyODK) but there is not a straight-forward process.
Are you using a visualization software, and can you connect the datasets together within that software instead of combining into one dataset?