Yes it is, see this post from the originator of the code.
https://community.ipfire.org/t/add-additional-sources-to-ipfire-blocklist-feature/8416/5
Not totally sure and not able to try and duplicate on my vm testbed at the moment, but if your entry was at the end of the sources list then you should not finish the line with a comma as in the line below.
If you put it after the ‘BLOCKLIST_DE’ list then you need to add a comma after the bracket at the end of the last line of that entry, just before where I presume you added your list.
'category' => 'attacker' }
If your list was added somewhere in the middle of the existing sources then you would need to show us the section involved plus some lines before and after the insertion.