Can I use my own function in a SQL update?

Ex: UPDATE IntranetUsers SET salt = CreateSalt(5)

When I try to run this I get 'CreateSalt' is not a recognized function name.

Am I going about this the wrong way?

I have a table that I need to update this field in. I need to use a function I created that generates a value and then store it in this field used by my encryption function.

TIA
Brad