Unfortunately no, you won't be able to summon seasonal characters off-banner. You will most likely have to wait until next Christmas before Dragonyule units become available again. They might release a special "Holiday/Seasonal" banner that holds all seasonal characters in the future but I wouldn't bet on it.
Question about Summoning Offbanner
So I was wondering, I just started playing and missed out on some adventurers, I wanted to know if for example, I wanted someone in the Dragonyule banner, would it be possible to pull them in another banner, like the recent Gala banner?
As Varien said, seasonal banners, like the halloween banner, dragonyule banner, and new years banner are limited, meaning that their units could only be pulled on the banners they were featured. However, if you wanted a unit from a non-seasonal banner from the past then you can pull them, though with a much lower pull chance.