In current Picard 2.0.4 this is not directly possible. You can set the iTunes grouping tag by script with e.g.:
But this will just change it, not reuse existing data. There is kind of a semi-manual workaround. You can create a new script in Options > Scripting (name it something like “Extend iTunes Grouping” with the following content (adapt to your needs by replacing “something to append” with what you actually want there):
$set(_id3:GRP1,%_id3:GRP1% something to append)
Now make sure to have this script disabled (no checkbox before the name) so it won’t run by default, because if it does it will just overwrite the iTunes grouping with only " something to append". Then when you have matched files to a release in the right pane select the files or the album, right click and use “Scripts…” > “Extend iTunes Grouping”.
The good news: With Picard 2.1 this story will be much improved. In Picard 2.1 there will be an option “Save iTunes compatible grouping and work”, which will allow you to use Picard’s builtin
grouping tag for this instead of the