Puwede, pero hindi advisable.
HTiB systems are designed to be low-cost and easy to use, but they are not designed to be upgradable.
HTiB amps and speakers were designed and tested to work together, so changing anything in the chain will not allow the system to work right.
The HTiB has an underpowered amp that will work best with the speakers they came with. Better speakers will not necessarily give you better sound. They might even overheat the HTiB's amp and kill it.
If you really want to try it, just make sure the impedance of the HTiB amp and new speakers match each other.
My advice, your next upgrade path should be an AVR + speakers. In the meantime, just enjoy the HTiB. I'm sure it's going to give you many more years of trouble-free use.