Using READTABLE function with 'TreatAsEmpty', Can I index which locations were changed?

Hello,

I have several excel files, that I am reading into Matlab to reformat and combine. These files are really messy and contain several strings {"NM", "NA", "ICE", & etc} intermixed with numeric values. I know I can remove all of these strings and treat them as empty when I use the readtable function, since I need the numeric value to in fact be numeric values (rather than strings containing numbers). However, I need to denote if that location actually contained one of those strings originally. Is there a way to index these for later combination? OR a better way to do this altogether?

Thanks, Melissa