Hexadecimal calculations are done by specifying a base-16 prefix for such numbers. Indeed, AddUp reads numbers expressed in any number base from 2 to 36. A number is in a base other than 10 when it is prefixed by its base value and a single quote. Base-2 (binary) numbers use a 2' prefix, base-8 (octal) numbers use an 8' prefix, base-3 numbers use a 3' prefix, and so on. For hexadecimal (base-16) numbers, this means using a 16' prefix before the number. So hexadecimal number FF (decimal 255) is written as 16'FF while hexadecimal 7C (decimal 123) is written as 16'7C.
Output of all AddUp calculations is given in decimal unless specified otherwise. To explicitly ask for hexadecimal output, start an expression with a prefix similar to what is used in front of hexadecimal numbers: use '16 at the start of the line. Notice how the quote is on the left side of the requested output base. This line prefix must be followed by a space to separate it from the expression to evaluate. The expression to evaluate could either contain all hexadecimal numbers or a mixture of numbers in various numeric bases, but the result will be presented in the number base that the line prefix specifies. The default base-10 applies when no base is specified.