You can use a SD-Card (64 GByte SDHC max) for that. The internal Android format utility (under Setup ==> Storage) needs to enable this. Should give ample space already.
I have seen one other brand providing another utility allowing both HDD and/or SD-Card to be formatted as Internal Android storage this instead of standard Portable Storage. Using a SSD would make it even bigger and perform well. A HDD is really too slow for this.