Can I add an infohash to a currently downloading torrent?

I expect the answer is no but:

I am currently downloading a file from a tracker site. 1 seeder, super slow, etc. But I'm trying to keep ratios up, trying to engage more with the site, etc.

But I found the same file on a public torrent site. I could download it fairly easily. Is there any way so somehow merge the two? Is this a no-no considering the possibility that the file is different, or maybe just flat out impossible because they may not be the same?

Thanks for your thoughts.