Hello! Thank you for your question Lu :D
Currently there is no built-in way to do this, because getting the behavior right is really tricky, and different projects will probably want different behavior. E.g. one project may want to use drag-to-select while another one may want to use ctrl + click.
I know App Inventor is currently investigating ways to add this behavior on-top of Blockly (i.e. handling drag-to-select outside of Blockly's gesture handling system). Here's the relevant
Google Summer of Code project. So you might want to reach out to them for their thoughts =) I know they also check this forum sometimes, so they may reply here. They've also expressed interest in providing it as a plugin once it's completed.
Sorry I can't provide more direct recommendations :/ But if you have specific questions I'm happy to help!
Best wishes,
--Beka