R imputation for strings in selected columns

I am struggling with multiple data-imputation packages in R and need your advice.

I have a data-set with 150.000 rows and 270 columns.

Every column has some missing data, but i need to make imputation only in 7 of them. But i need all the columns to be considered during the filling of those 7 columns. No numerical data, only strings.

What is the best package to use, in your opinion, and do you know how to code it?

If the names of columns are: A, B, C... X, Y, Z.

And we need to make an imputation only for strings in N and M, but based on all of the columns.

Many thanks in advance.