If you have a US passport, according to TIMATIC, you can get a visa on arrival in Tanzania (must be paid in cash with US$50 / US$100 notes). Visa on arrival may require proof of an onward ticket, but that might be easier to arrange -- by the time you've bought your ticket, maybe you'll have a return ticket too.

Proceeding under the assumption that you do not hold a US passport or that the visa on arrival won't work, you can definitely use a refundable ticket for this. All tickets purchased from UA are fully refundable for the first 24 hours after ticketing, which may be useful for visa-on-arrival but less so if you need to send in paperwork.

As already suggested, award tickets are refundable with the payment of the appropriate fee based on your status level and how far in advance you cancel ($0-$125).

If you don't have enough miles for an award ticket, you can certainly purchase a refundable ticket, but make sure you read the fare rules extremely carefully.

While UA sells refundable* tickets on, say, IAD to DAR, their fare is over $7000 roundtrip. TK and ET are both selling refundable fares in the $2600-$2700 range (or possibly less; I just checked for full Y). You may want to call one of those airlines or consult a travel agent to make sure that you booked the product that you want, and double-check the receipt to ensure that it says it's refundable.

Hope this helps.

* If, for whatever reason, you really want to loan UA this kind of money -- they call them "unrestricted" fares in the advanced search tool. They also offer "flexible" fares, which may allow a full refund or may charge an extensive change / cancellation fee; you'd have to read the rules to be sure.