Is it possible to convert a regular passenger plane to cargo?
It is not. A Feature to covert was announced but not implenented.
Full natural conversion (eg turning a 738 into a 738-BCF), no
But you can always fly some planes seatless and it will be cargo only flights. The CU capacity will be most efficient if it’s done with a type that naturally “converts” such as the 737-700C (and others), but it can be done with any plane that only holds cargo for less efficient capacity.
I would not recommend it though, under most circumstances.