Are you looking for something like this?
library(tidyverse)
testdat <- tribble(
~SUBJNO, ~X, ~Y,
'CAN1', 96.2, 2.52,
'CAN2', 85.0, 4.28,
'CAN3', 89.5, 3.67,
'CAN4', 98.1, 3.33,
'CAN5', 89.7, 3.34,
'CAN6', 84.3, 3.55
)
newdat <- testdat %>%
mutate(SUBJNO="Cancer Patient")
newdat
#> # A tibble: 6 x 3
#> SUBJNO X Y
#> <chr> <dbl> <dbl>
#> 1 Cancer Patient 96.2 2.52
#> 2 Cancer Patient 85 4.28
#> 3 Cancer Patient 89.5 3.67
#> 4 Cancer Patient 98.1 3.33
#> 5 Cancer Patient 89.7 3.34
#> 6 Cancer Patient 84.3 3.55
Created on 2019-12-03 by the reprex package (v0.3.0)