Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
As Pommie, says, it all depends on what the other pins are doing, you may well be able to better utilise what you've got.
Even if you can't, you can't multiplex a single pin, you need multiple pins to feed the multiplexer/port expander - so you would need to move some of your existing I/O's to the multiplexed connections.