The easiest way to make an assignment extra credit is to simply create a new assignment as you always do and give it a point value of zero. Then, anyone who does not do it will not be penalized, while those who do will receive any points you give to them. Canvas will automatically add any points students earned on the new assignment to the TOTAL grade or (in the case of adding an Assignment within a weighted group) to that specific assignment group.
However, there a several additional things to consider, especially if you are weighting your assignments by groups. Please check this Canvas help screen for more information: How do I give extra credit in a course?