Just need to import them as you would all the way back in Dynamo 2.0 (and likely before that too).
ie: import clr, which imports the common language runtime into Dynamo’s Python integration. This presentation has a good breakdown: dynamoPython/DivingDeeper_ABeginnersLookAtPythonInDynamo_AU_London2018.pdf at master · Amoursol/dynamoPython · GitHub
Another slightly more complex example from this thread:
import sys
sys.path.append(r'C:\Program Files (x86)\IronPython 2.7\Lib')
import ctypes
which adds the ctypes library to the associated Python node.