If I am remembering the scenes where Null manifested powers correctly, it seemed that he gained the power when he really wanted it, such as in a life-or-death situation, hence why there is a long-standing theory that Null is actually a mind-wiped David Haller, and his actual power is power manifestation, hence explaining the Shadow King's interest in Null even before his powers manifested and the Shadow King's confusion in the end of chapter event. Null would be able to get new abilities from other heroes if he really wanted them. Still, he would actually need to consciously try to manifest them individually, and attempting that also seems to require extreme events for them to actually manifest. However, this isn't universal, considering Null was able to develop low-level X-ray vision and minor shape-changing without any real threat being present, but the canon status of those abilities are in question, and there is probably a lore explanation for that that I missed.
What is clear is that mutant girls he is close to seem to have the easiest powers to copy, as he is already familiar with their abilities due to constant exposure, giving needed context to form a facsimile power and probably x-gene shenanigans.
Again, this is based on my reading of the scenes, and that should be taken with industrial accident levels of salt. So yes, Hulk abilities seem possible, but he would need to try to manifest those powers individually in extreme life-or-death situations, which isn't a good idea.