I am not sure what you mean by "at the input step" - what is your data source (e.g., Excel file, DB connection, etc.)? Are you saying they are not coming into your Input Data (or equivalent) tool?
If you mean you had fields that came in from your Input Tool that you mistakenly removed at some point, adding them back will depend on how / where they were removed. For example - Did you manually de-select them from a Select tool? A Join tool? Not include them in either the key / data fields for Transpose / Cross Tab? Fields can be dropped from a dataset in a number of ways, so it would be difficult to give you a pointed recommendation without more detail.
That said, once you know where you dropped or de-selected them, you should just be able to adjust the configuration of that tool so as not to remove them. You might have to update some downstream tools as well depending on your workflow (e.g., Summarize, Unique, etc.). After running your workflow, you can click each tool's output anchor(s) to see the data within the Results pane that was processed by that tool. So, if you don't know where you dropped them, just run your workflow and start from the very beginning input tool and click through each successive tool's output to find out where they got dropped. Most of the time, this can be quickly determined from looking at the tool's configuration. If nothing else, you can compare the input vs. output anchor(s) of each tool in the Results pane to visually see where your fields are getting dropped.
If I misunderstood your question, please provide some clarifying detail (e.g., additional explanation, screenshots, draft workflows, etc.) around what you are trying to do and I'm happy to try and help!
