I don't think it can. Not easily, anyway. Because input method is managed by the OS, right? I don't think an app could have any control on it even if it wanted to
If you don't mind it, there's always the shortcut LeftAlt-RightShift, in Windows, that switches input methods quickly. There's probably something similar in Mac OS.
The only other way I see would be to set up a macro that would bind your Tab key to change input methods as well. You'd have to disable it once you're done working. Not too practical.
Oh yeah, and if it's okay for your whole column to be in hiragana only, you can write text in the Japanese column in romaji, and once you're done, select it all and run a Find and replace for every syllable. I know, it's a lot of characters. But well.
If you were good in programming, you could set up a tool that would automatically run that replacement in Excel. There are various tools on the Web that do it for text you input. There should be a way to port it to Excel, but... I don't know it.
There might even be some tools that can intelligently-ish put kanji in. But again, they'd need to be ported to Excel.
Sorry I can't be of more help.