import clr
clr.AddReferenceToFileAndPath(r"C:\Folder\Subfolder\file.dll")
is the simplest way as proposed by Jeff in the comments. This also works:
import clr
import sys
sys.path.append(r"C:\Folder\Subfolder") # path of dll
clr.AddReference ("Ipytest.dll") # the dll
import TestNamspace # import namespace from Ipytest.dll