If I've understood correctly, this is exceedingly possible!
# set up data
dat = data.frame(M1 = 1:5, M2 = 1:5)
dat
#> M1 M2
#> 1 1 1
#> 2 2 2
#> 3 3 3
#> 4 4 4
#> 5 5 5
# combine columns
dat$M3 = dat$M1 + dat$M2
dat
#> M1 M2 M3
#> 1 1 1 2
#> 2 2 2 4
#> 3 3 3 6
#> 4 4 4 8
#> 5 5 5 10
# drop old ones?
dat$M1 = NULL
dat$M2 = NULL
dat
#> M3
#> 1 2
#> 2 4
#> 3 6
#> 4 8
#> 5 10
# tidyverse
library(tidyverse)
tibble(M1 = 1:5, M2 = 1:5) %>%
mutate(M3 = M1 + M2) %>%
select(-M1, -M2)
#> # A tibble: 5 x 1
#> M3
#> <int>
#> 1 2
#> 2 4
#> 3 6
#> 4 8
#> 5 10
Created on 2021-12-17 by the reprex package (v2.0.1)
Next time, please provide a reproducible example.