How to add a line in sed if not match is found

Question

I am using the following sed command to replace some parameters in a config file:

sed -i 's/^option.*/option=value/g' /etc/fdm_monitor.conf

Now I have one problem. If the line does not exist, I want to add it to the bottom of the file.

I am calling this with a popen out of a C program. I tried using awk.