Is totally possible: you don't need any special license.
The only think, as you probably already know, to be a plumber is a very local job. If you need to interact with local people, you need a good spanish level.
Another option, is just to work for expats. The community is big, But I don't know if big enough to give you work periodically.
I recommend you to ask on this group of FB: https://www.facebook.com/groups/barcelonaexpatsgroup/