Try to use system() function:

awk '{printf("%s ",$1); system("d2h " $2)}' file

In your case system will call d2h 247808 and then append output of this command to printf output:

Mike 3C800

EDIT:

As system uses sh instead of bash I can't find a way to access .bashrc. But you can still use functions from your current bash script:

#!/bin/bash
d2h() {
    # do some cool conversion here
    echo "$1" # or just output the first parameter
}
export -f d2h
awk '{printf("%s ",$1); system("bash -c '\''d2h "$2"'\''")}' file

EDIT 2:

I don't know why, but this is not working on my Ubuntu 16.04. This is strange, because it used to work on Ubuntu 14.04.

Try doing this :

awk '{print $1 ; printf "%x\n", $2}' file

AFAIK, you can't use a bash function in awk but only a script. You can use an awk function if needed.

You can call bash from awk and use its output. That is obviously dangerous from a performance perspective if it happens too often. Quoting the man page:

command | getline [var]

Run command piping the output either into $0 or var,

command would be a bash script that contains the function definition and executes the function.

Using a user defined bash function inside awk

Disclaimer: I realize this is not what the OP is trying to do, but Google will lead others like me to this answer.

Situation

You have a bash script is organized with functions (because you do not hate yourself or [most] coworkers) and at least 1 of those functions needs to call another from within awk.

Solution

Script

#!/bin/env bash

# The main function - it's a sound pattern even in BASH
main(){
    # In the awk command I do some tricky things with single quotes. Count carefully...
    # The first $0 is outside the single quotes so it is the name of the current bash script.
    # The second $0 is inside the single quotes so it is awk's current line of input.
    awk '{printf("%s. ", ++c); system("'$0' --do"); print $0}'<<-PRETEND_THIS_IS_AN_INPUT_STREAM
        and
        and
        well
    PRETEND_THIS_IS_AN_INPUT_STREAM
}

# functionized to keep things DRY
doit(){
    echo -n "doin' it "
}


# check for a command switch and call different functionality if it is found
if [[ $# -eq 1 && $1 == "--do" ]];
then
        doit
else
        main
fi

Output

$ ./example.sh
1. doin' it and
2. doin' it and
3. doin' it well