You can easily migrate existing files into a DB pool. Take a look at this KB article; post back if the procedure is not clear (hint: you may need to create a temporary mount point using Window Disk Management or the tool of your choice).
Faced with your situation I would create a DB pool with just the 3TB disk and then move the files as described in the article. Because it is a move (vs copy) the actions will take place very quickly. Then add the empty 5TB drive to the pool. In the end you will end up with a two disk, 8TB pool containing all of your old files.
DB may have a procedure to automate existing file migration at the time of pool creation. It's been a while since I created a new pool on a populated drive. Regardless, I generally use the above approach as I understand and can control the process.