Hi Sebastian,
No this is not possible. The portfolio EA is comprised of many individual EA strategies and each could have different rules for long only, short only etc BUT at the portfolio level you cannot just change this on all of the EAs.
Also you should not want to be able to do this as each EA should be tested individually and this may not be beneficial for many of the strategies in a portfolio.
The portfolio EA is only really to make the final trading part easier and have to load less EAs on less chart.