Okay, I guess that you need something to compress an existing expession. If you're willing to specify the summand, index, and index range, then someFunction could subtract the terms and add the (unevaluated) Sum of those terms.
convert_to_Sum:= (ex, summand, index)->
simplify(ex - sum(summand, index) + Sum(summand, index))
;
Example:
ex:= A[0]+A[1]+A[2]+A[3]+A[4]+A[5];
convert_to_Sum(ex, A[i], i= 0..5);