Is it Possible to Add a Charity as a Venture Partner and Are There Legal/Tax Issues?

Potentially, however you will need to be careful of potential conflicts of interest. These may need to be managed via an additional agreement - otherwise it may not be a good idea to proceed. 

If the charity provides separate additional legal agreements, we would strongly recommend having an expert fund attorney review before signing anything. There may be implications for your Fund (or for tax purposes) that may arise.