Depending on how you want to specify it you can do it like the below. Let me know if this solves your problem. I simply went for a transpose with a cbind.
df <- data.frame(
Player = c(1, 2, 3),
Age = c(3, 45, 18)
)
BMI <- data.frame(
Player_1 = 23,
Player_2 = 34,
Player_3 = 24
)
df2 <- cbind(df, BMI = t(BMI)) #you can do t(BMI) but then the name in the data.frame will be the same as the command.
df2
#> Player Age BMI
#> Player_1 1 3 23
#> Player_2 2 45 34
#> Player_3 3 18 24