Replacing missing with string in excel file using mata

[Neko Novi]

Hi all,
i create some excel tables in stata, using putexcel and mata xl() command, but since i do it for several different folders it is all in loops over folders. Problem is that i want in each table to replace missing with string value NA, and what i understand it can be done with mata command xl() put_string option, but since i would loop it over each cell, i have mata loop in stata loop and it doesnt work. Then i wanted to create function in mata that will handle it, but i cant get it done as xl() uses structures and i in that point i get lost and got errors. What am I doing wrong in the example i will put below?. Additionally, if there is some completely different and simpler way to do the same thing i would appreciate advce!

mata: mata clear
mata
void add_na(real scalar a, real scalar b) {
for (r=a; r<=b; r++) {
   for (c=1; c<=3; c++) {
   struct mystruct scalar e
   cc = c*2
   rows = (r,r)
   cols = (cc,cc)
   rows
   cols
   cells = e.get_number(rows,cols)
   if (cells == .) {
        e.put_string(r,cc, "NA")
   }
}
}
}
end

foreach f in folder1 folder2 ... {

cd "each folder1/2/3... where i create xlsx"
//here i put numbers into table etc, with "putexcel" .... using the same example.xlsx

// here i use some xl() things for which i do not need loop

mata b = xl()
mata b.load_book("example.xlsx")
mata b.set_column_width(2, 2, 15)
mata b.set_column_width(3, 3, 10) //etc

// here i would like to replace all missings with string NA
mata ad_na()

}
//