We have a multi-author conference paper. What to do if one co-author does not respond when extending it to a journal paper?
Check this with the journal you want to submit to. If you don't actually need them to submit this as a journal paper, this is what you'd ideally do:


Let them know you're writing an extension
Write the extended paper
Send the paper to them with a note saying "Unless you let us know otherwise, we plan to submit to journal X at date Y."
Submit, briefly explaining the situation to the editor.