Not only, but also (in cantonese) 佢唔單只感冒，仲發燒添。
  Not only does he have a cold, but he also has a fever. 


What is the function of 添？ I feel rather awkward saying it. I searched the dictionary and doesn’t look like it has added any meaning.
You might be interested in these notes from ABC Cantonese on the word 添:


  1 more 
  
  畀多我一百蚊
  bei2 do1 ngo5 jat1 baat3 man1 tim1
  Give me one hundred dollars more 
  
  食多啲
  sik6 do1 di1 tim1
  Eat some more 
  
  Notes: occurs at end of sentence to indicate action is expected to be repeated; usually pronunciation with high falling tone; also written as 㖭 tim1, 添 tim1
  
  2 as well, also, more, again 
  
  佢撞到人唔道歉,仲鬧人
  keoi5 zong6 dou2 jan4 m4 dou6 hip3, zung6 naau6 jan4 tim1
  After bumping into somebody, he didn't apologize, and even scolded that person as well 
  
  噉做法仲弊
  gam2 zou6 faat3 zung6 bai6 tim1
  Doing it this way is also even worse 
  
  Notes: follows 仲 zung6 to emphasize something additional is involved