Welcome to the Forum! Actually, you can add custom indicators in FSB Pro, and not in EA Studio. The indicators in EA Studio are added from the Forex Software LTD, however, if you wish to have a precise indicator, let me know, and I will contact them about it. They will put it on their to-do list, but normally they do not guarantee a time when they will place it in the program.
The idea of EA Studio is to work FAST, that is why it uses the indicators of Meta Trader. Also, when people put custom indicators(as it is in FSB Pro) there might be issues with the indicators. From the code of the indicators. And then normally the traders were blaming the FSB Pro work and the EAs, but the issues were coming from the custom indicators. Just sharing with you why it is not available right now.