Yes, they can be used and if the cracks aren't to bad they won't leak for a while, but they will leak and the crack/s will get worse. Even if they aren't cracked out of the box they will crack sooner or later, usually sooner. One immediate thing you can do is remove the tube and turn it over, use the top for the bottom, that will buy you some more time with them.
What our ultimately want to do if you want to keep them long term is replace the original tube. I use a 3ml BD Syringe tube, cut it to size and I'm done, much more durable and will last indefinitely. There are places you can buy replacement tube that are said to be more durable. Gotvapes sells them but I have no experience using those so I can't vouch for how long they will last.