Usage of "danwei" in english
looks through Google Books and what not comes up with a few instances of danweis:

From Family to Market: Labor Allocation in Contemporary China


  This danwei-based income discrepancy worsened drastically during the reform years as the danweis acquired more autonomous authority in allocating financial


Globalisation and the Chinese City


  Similarly, 'commercial land' refers to the lots occupied by the danweis under the administration of the financial or commercial system (Huang 1997).


上海新城: 追寻蔓延都市里的社区和身份


  Danweis ('danwei' literally means 'work unit') were cooperative or state-run companies whose members lived and worked communally, and often practiced one ...


The Chinese Dream: A Society Under Construction


  15 The new Constitution banned all land transactions and land was allocated to danweis* free of charge. 


I would totally go with a pluralized danweis for multiple danweis.