Yes, you can hold by trading. I've been doing this since my altcoins are all in red flag. But I see to it that if I sold it I can buy the same altcoins at lower price to increase the amount of my altcoins, thus increasing portfolio as well.
This is I guess a common practice of traders to keep holding their coins when they are sure for not making some good money with them. If the prices are lower, they tend to collect more and more in order to increase their variety of coins. For trading, you need a price that is much higher than the one at which you bought your coins. Otherwise trading would be done but the trader will face a good loss.