How are fellowships typically funded?
My answer assumes the US university system, based on your stated location

Endowed funds (for positions or otherwise) are quite common at universities, so every University has an "Endowments Office" or "Office of Giving" or the like, which will most happily sit down with you to work out these sorts of details.

You will definitely need to sit down with that office to determine the details, however, because it will likely be different for every university. Factors influencing these details will likely include: public vs. private, overhead rates and structure, how the university manages its endowments, state and local tax laws, etc. 

In short: get the university's experts involved.  The sorts of sums that you are considering will certainly get their attention.