I'm very excited about the brand new Dataset function. I have played with it in Wolfram Cloud, and haven't figured out how can I add a new column into an existing Dataset. Here is an example:

data={<|"col1"->1,"col2"->2|>,<|"col1"->3,"col2"->4|>,<|"col1"->5,"col2"->6|>};
ds=Dataset[data]

Now I can play with ds columns. For example, I can easily make calculations between columns using they names like:

ds[All, (#col1+#col2&)]

{3, 7, 11}

Another way is:

ds[All, <|"col3"-> (#col1+#col2&)|>]

<|col3->3,col3->7,col3->11|>

Now, how can I update ds, to append the brand new calculated column as "col3"?

I tried Join[ds,ds[All, <|"col3"-> (#col1+#col2&)|>],2] without success.