Is there a way to use Electrum to double-spend a transaction that is not RBF enabled?
The transaction was made by another wallet that has no RBF support.
I've imported the relevant private key into Electrum and see the pending transaction, but is there a trick for me to cancel it even though I don't have the right-click menu option available due to the absence of the RBF flag?
It's impossible for you to use electrum for a transaction that has no RBF support once you have broadcast your transaction. You can use CPFP. It's there in Electrum wallet in order for your transaction to be added to the next block. Electrum can be used for transactions supports RBF.