Can I use W-2 Box 14 if the tax provider software doesn't support a given Box 12 code?

[bn2_]

If W-2 box 12 in the tax provider software doesn't support a given code (ex. code FF), can you just list it in Box 14 if it's currently unused?

[SteamTrain]

Not really proper..so I doubt it.

Just Use software that does include it.

TTX does have box 12 code FF in both the Online and Desktop software.   With the "Online" software, if you already have used one (or more ) box 12 code(s), you just click on the "+ Add another row" just below the box 12 entries you do have.

[Critter-3]

I agree ... TT certainly has that option ...