Actually, there's a workaround to entering manually! You only need to mail in the substitute 1099-B statement provided by Prosper—along with Form 8453. The substitute statement contains all of the transactions that need to be reported in an IRS-sanctioned format (it meets the IRS requirements for a 1099-B substitute form).
I just found out about this and I'm so glad I don't have to enter 20+ pages of transactions...
