Bash: adding a key/value pair to an existing config file

I have an existing config config.ini file with the following content:

VALUE_A=aVALUE_B=b

Using Bash, I'd like to add a new key-value pair VALUE_C=c to get the following:

VALUE_A=aVALUE_B=bVALUE_C=c

Is there a concise way to do this with Bash (ideally a one liner)?