Hi,
I checked the code of the library we're using for that and it doesn't seem to support ceil.
However, I think you could add that easily.
In the file administrator/components/com_hikashop/inc/expression.php you have this list of supported PHP functions:
var $fb = array( // built-in functions
'sin','sinh','arcsin','asin','arcsinh','asinh',
'cos','cosh','arccos','acos','arccosh','acosh',
'tan','tanh','arctan','atan','arctanh','atanh',
'sqrt','abs','ln','log');
So I think you could just add the functions you need in there providing they only use one parameter.
And while you can't cast numbers into integers, you can use the intval() function for that so you could also add in that same array of allowed functions.
Modulo is supported since you can use the % operator.
Let us know how it goes so that we could add these extra functions to the list so that you won't have issues updating in the future.