Un-expand a column

I have a dataset uploaded from SQL Server which links lots of tables together. I have created a new table by referencing one. One of the columns links to a linked table and I already expanded it to just one field from those listed. I have now found I need another field from that linked table. How do I add that table back in as a column or how do I pick an extra field to the one I have already picked. I tried to remove the expand step in my steps but that just crashed the system as it caused issues with later steps where it wanted to reorgniase the field order (the next step to expanding). Not very easy to explain but I cant seem to find a way of getting the field I want. When i try to add new column it isnt listed and if I type the field name in square brackets under custom column it comes up with error in the rows not value as I would expect.