Assuming file names to be coded as follows: data_001.xlsx, data_002.xlsx, .., data_100.xlsx, you could build a solution based on the following approach
nFiles = 100 ;
for fId = 1 : nFiles
filename = sprintf( 'data_%03d.xlsx', fId ) ;
num = xlsread( filename ) ;
if fId == 1
buffer = num ;
buffer = buffer + num ;
Note 1: this assumes that there is only one column of numeric values in the file. If there are multiple, you have to select the relevant column, e.g. if it's column 3: buffer=num(:,3) and buffer=buffer+num(:,3).
This approach can be extended a little with code for e.g. taking all files from a specific folder, or matching names if they can be stored in various orders.