If I understand correctly...

• Enter the formula in the cell normally.
• In the formula box dropdown, select Convert Formula to Text in Cell

This will replace the formula result with the formula.

Warning: This will replace the formula with text so if any other cell's formula is dependent on the converted cell it will break.

Using an AppleScript to Copy Formulas

A more reusable approach would be a simple AppleScript that replicates the Copy command but only copies the bare formulas:

on join(cells, delimiter)
  set previousDelimiters to AppleScript's text item delimiters
  set AppleScript's text item delimiters to delimiter
  set resultString to (cells as text)
  set AppleScript's text item delimiters to previousDelimiters
  return resultString
end join

on getSelectedFormulas()
  set tab to character id 9
  set lf to character id 10

  tell application "Numbers"
    tell document 1's sheet 1's table 1
      tell selection range
        set {min, max} to {column 1's address, column -1's address}
        set formulaRows to {}
        repeat with aRow in rows
          set formulaCells to {}
          repeat with aCell in aRow's cells min thru max
            set formulaCells to formulaCells & (aCell's formula)
          end repeat
          set formulaRows to formulaRows & ¬
            my join(formulaCells, tab)
        end repeat
        return my join(formulaRows, lf)
      end tell
    end tell
  end tell
end getSelectedFormulas

on run()
  tell application "Numbers"
    set the clipboard to my getSelectedFormulas()
  end tell
end run

(Note: I’m the author of the script; it’s under the ISC open source license.)

This also eliminates the need to modify the original formulas.

Binding the script to a keyboard shortcut

In order to bind this script to a keyboard shortcut in Numbers, open Automator.app to create an Automator workflow. Choose the option Service receives no input in Numbers, create a single Run AppleScript step and paste the AppleScript code there.

Save the workflow under the name Copy Formulas, then open System Preferences and click on Keyboard, then Keyboard Shortcuts » Services » Numbers » Copy Formulas. Configure a keyboard shortcut; I’d recommend ⌥⇧⌘C because you’re probably going to use it often with its counterpart, Paste and Match Style, which already has the shortcut ⌥⇧⌘V.