You didn't answer Steve's question: do you mean hex as in a string of ASCII characters? If so, then you first need to convert the characters (ASCII codes) to binary (16 bits).

If not, then you're probably confused about the difference between binary & hex numbers: there is none!

A number is a number, and you can represent it in any form you like; eg,
* Roman Numerals
* Binary (base 2)
* Ternary (base 3)
* Octal (base 8)
* Decimal (or denary; base 10)
* Hexadecimal (base 16)

Now, inside the CPU, numbers are always represented in Binary (bits), since it relies on digital electronics which can only ever be in one of two states. However, writing numbers in binary is tedious for people, so we often use Hex instead, which is quicker and convenient, as 1 Hex digit corresponds nicely to 4 bits

So, don't worry about hex; just think about how you add columns of digits & cope with a carry in any radix!