Using the "IF" function to create a new column

Ah, OK, sounds to me like you want more of a "lookup" function. See below code for creating one of these. Note that while I use a manually created list, you could use a list created from something like a database table, etc. To create this example, just create a blank query and paste in the code. Then you can test it with in the Query Editor interface. To use it, just create a new column that calls the function name with a parameter:

let
    fnHex2Dec = (input) =>
 
let
 
values = {
 
{"0", 0},
{"1", 1},
{"2", 2},
{"3", 3},
{"4", 4},
{"5", 5},
{"6", 6},
{"7", 7},
{"8", 8},
{"9", 9},
{"A", 10},
{"B", 11},
{"C", 12},
{"D", 13},
{"E", 14},
{"F", 15}
 
},
 
Result = Value.ReplaceType({List.First(List.Select(values, each _{0}=input)){1}},type {number})
 
in
 
Result
in
    fnHex2Dec

This function takes a single input parameter, a single text character and translates it to a decimal equivalent. You can test this function by clicking the "Invoke" button in the Power Query Editor window. Be sure to enter a single value preceded by a single quote, such as 'A. The single quote forces the input to be recognized as text. This ensures that if you enter a 7, that it is recognized as text instead of a number.

This is "M" code by the way.

To use it, create a new column in the Query Editor like:

=fnHex2Dec("A")