Is it cryptographically possible to have aggregate signature schemes that can improve privacy in UTXO consolidation? My understanding of Schnorr signatures, Taproot and Graftroot and how they improve privacy in smart contracts / multi-sig is by allowing users to aggregate signatures so the individual signatures and keys are not visible but summed in the aggregate key / sig. 

Are there any cryptographic protocols that allow the summing of UTXO inputs? So for example multiple inputs in a transaction were summed to one aggregate input and then signed with the sum of the input private keys? 

Is it possible to improve privacy of UTXO consolidation?
Yes, I believe that the accumulator constructions as described here are a good candidate. The same idea applied to Bitcoin is implemented by Tadge Dryja here