Something like this works, but you lose the convenience of @transform:
transform(df; Symbol("newcol") => cos(df[:oldcol]))
You can also use df[Symbol("newcol")] = df[:oldcol] instead of transform (which is in DataFramesMeta, not in DataFrames BTW).
I’m not sure whether this has been discussed in DataFramesMeta yet, but you could file an issue about it, as it should be possible to support this in @transform in some way (maybe it already exists).