You cannot relicense BSD code. In general, you cannot change anything about the copyright or license for any work that you do not own the copyright to.
Perhaps you mean the BSD licensed artifacts are "compatible" with copyleft licenses?
Relicense might not the best word but compatible also doesn't cover it in this context.
Saying you can "assimilate" BSD licensed code in to a GPL project might be more appropriate. Since rights such as "Redistribution and use in source and binary forms" which the BSD gives you no longer apply after the code is in a GPL project [1].
Perhaps you mean the BSD licensed artifacts are "compatible" with copyleft licenses?