The answer of bobthechemist seems to work fine as long you have no named rows and only named columns as in (col1, col2 col3). If you have both named column's and named rows it slightly different. I take the same example of bobthechemist but now with named rows too. So instead of data= {<|"col1"->1,"col2"->2|>,<|"col1"->3,"col2"->4|>,<|"col1"->5,"col2"->6|>}

I have

data = <|"1" -> <|"col1" -> 1, "col2" -> 2|>, "2" -> <|"col1" -> 3, "col2" -> 4|>,"3" -> <|"col1" -> 5, "col2" -> 6|>|> 

so the rows have row number 1 to 3. Notice too that the bracket's in data are removed (strangely enough you need then if you have no row names but with row names you have to omit them). Now you can append col3 being the addition col1 and col2

ds3 = Dataset[data] ds4 = ds3[All, Append[#, "col3" -> #col1 + #col2] &]

The answer of bobthechemist seems to work fine as long you have no named rows and only named columns as in (col1, col2 col3). If you have both named column's and named rows it slightly different. I take the same example of bobthechemist but now with named rows too. So instead of

data={<|"col1"->1,"col2"->2|>,<|"col1"->3,"col2"->4|>,<|"col1"->5,"col2"->6|>} 

I have

data = <|"1" -> <|"col1" -> 1, "col2" -> 2|>, "2" -> <|"col1" -> 3, "col2" -> 4|>,"3" -> <|"col1" -> 5, "col2" -> 6|>|> 

so the rows have row number 1 to 3. Notice too that the bracket's in data are removed (strangely enough you need then if you have no row names but with row names you have to omit them). Now you can append col3 being the addition col1 and col2

ds3 = Dataset[data]; ds4 = ds3[All, Append[#, "col3" -> #col1 + #col2] &] 

The answer of bobthechemist seems to work fine as long you have no named rows and only named columns as in (col1, col2 col3). If you have both named column's and named rows it slightly different. I take the same example of bobthechemist but now with named rows too. So instead of

`data={<|"col1"[Rule]1,"col2"[Rule]2|>,<|"col1"[Rule]3,"col2"
[Rule]4|>,<|"col1"[Rule]5,"col2"[Rule]6|>}

I have

data = <|"1" -> <|"col1" -> 1, "col2" -> 2|>, 

"2" -> <|"col1" -> 3, "col2" -> 4|>, "3" -> <|"col1" -> 5, "col2" -> 6|>|>

so the rows have row number 1 to 3. Notice too that the bracket's in data are removed (strangely enough you need then if you have no row names but with row names you have to omit them). Now you can append col3 being the addition col1 and col2

ds3 = Dataset[data] ds4 = ds3[All, Append[#, "col3" -> #col1 + #col2] &]

The answer of bobthechemist seems to work fine as long you have no named rows and only named columns as in (col1, col2 col3). If you have both named column's and named rows it slightly different. I take the same example of bobthechemist but now with named rows too. So instead of data= {<|"col1"->1,"col2"->2|>,<|"col1"->3,"col2"->4|>,<|"col1"->5,"col2"->6|>}

I have

data = <|"1" -> <|"col1" -> 1, "col2" -> 2|>, "2" -> <|"col1" -> 3, "col2" -> 4|>,"3" -> <|"col1" -> 5, "col2" -> 6|>|> 

so the rows have row number 1 to 3. Notice too that the bracket's in data are removed (strangely enough you need then if you have no row names but with row names you have to omit them). Now you can append col3 being the addition col1 and col2

ds3 = Dataset[data] ds4 = ds3[All, Append[#, "col3" -> #col1 + #col2] &]