a 256-bit number is just a number. The BigInteger class has several constructors that take different objects. How is your number stored? Probably the simplest would be to convert it to a string and use that.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors
"budha", may have missed that we have a policy on screen names here at JavaRanch. Basically, it must consist of a first name, a space, and a last name, and not be obviously fictitious. Since yours does not conform with it, please take a moment to change it, which you can do right here.