The code you're using to append elements to t is wrong, you should do in this way:

t <- list()
for(i in 1:length(net)) { 
  a <- cbind(imp.qua.00.09[i], exp.qua.00.09[i], net[i])
  t[[length(t)+1]] <- a
}
t

Your code is wrong since at each step, you transform t into a list where the first element is the previous t (that is a list, except for the first iteration), and the second element is the subset. So basically in the end you're getting a sort of recursive list composed by two elements where the second one is the data.frame subset and the first is again a list of two elements with the same structure, for ten levels.

Anyway, your code is equivalent to this one-liner (that is probably more efficient since it does not perform any list concatenation):

t <- lapply(1:length(net),
            function(i){cbind(imp.qua.00.09[i], exp.qua.00.09[i], net[i])})