How do I add a column to each data frame in a list

Here is an approach with base R only:

lapply(mylist, transform, ratio = y / y[1])
# [[1]]
#   time y ratio
# 1    1 2   1.0
# 2    2 3   1.5
# 3    3 6   3.0
# 
# [[2]]
#   time y    ratio
# 1    1 3 1.000000
# 2    2 4 1.333333
# 3    3 7 2.333333

It might be easier to understand when written as

lapply(mylist, function(x) transform(x, ratio = y / y[1]))

Also, see ?transform.