You could potentially copy/paste the actual cells vs. their content. Here's what I mean:

CopyToClipboard[NotebookRead[Cells[NotebookObject[obj],CellStyle->"Text"]]]

Then Paste:

Paste[]

The Text cell style renders properly outside of the notebook front-end, and you'll need to convert all your Input cells to Text cells first, but that is pretty easy to do: you can alt+click on an input cell in the notebook and it selects all of the cells of that style. Then, you can do ctrl+7 to turn them all into text cells.

Hope this helps!

obj is just the name of the notebook file being represented as a NotebookObject. It is assuming you are trying to extract the unevaluated code from another notebook.

But I guess you're copying content from outside of Mathematica into a Mathematica notebook? I tried to recreate your issue by copying a column from a Google sheet, and using Paste[] gave me each spreadsheet cell as an Input style cell in Mathematica, without them being added together—I can't seem to recreate your issue.

Can you post a picture or step-by-step process of what you are doing?

I find this very useful to copy data from Excel to Mathematica:

https://mathematica.stackexchange.com/a/14659/251

It is a VBA script for Excel that does the conversion automatically when you copy with a modifier key (CTRL+SHIFT+C instead of CTRL-C)

This works if you copy into the clipboard using Mathematica. The issue comes when you copy outside of Mathematica. For example, if you copy a column of numbers from a spreadsheet and then evaluate Paste[], then Mathematica multiplies those numbers together, rather than just displaying them as a string with tabs.