Hi @Anonymous, yes you can add custom columns such as this. It can be done easily in the Query Editor or in DAX.
This can be useful to SUM to get numbers that represent Counts.
You can also do this using calculated measures using the COUNTROWS function, but it depends on how confident you are with creating DAX measures. If you have a particular issue you are trying to solve, post more about it here and we will do our best to help you.