Is there a way to allow use of unconfirmed RBF outputs in transaction building? Bitcoin Core seems to not consider unconfirmed outputs that are marked replaceable in input selection. Is there a way to override this protection and allow use anyway? The transaction is created using bitcoin-cli.

Edit:
Turns out the unconfirmed output was not change but came from an external wallet. Bitcoin Core doesn't use unconfirmed external outputs.
Bitcoin Core will happily spend RBF marked inputs it doesn't avoid them much less prohibit them.

Perhaps you're being confused by the fact that it will not spend an unconfirmed input created by a third party? Or maybe you have spendzeroconfchange set off or only have long-chain unspents?