Unsupported formats might work sometimes, but then not again. People edit H.264 and it seems fine, but then it stops working. Or you export and everything is wrong. Because FCP isnt designed to work with those formats. The problem is, that it SEEMS like it will, and it might, once, or twice. Or all the time you are editing and then you go to export and...it doesn't work anymore.
If you use FCP, and you want it to work right all the time, you need to convert to FCP media. If this is media that your job at that office needs you to edit, then they should buy the tools that allows it to be converted...or the software you can actually edit it with. it's an easy conversation.
"Hey, we want you to edit this video."
"OK...oh, wait. I can't edit this particular video with the software we have. It needs to be converted. And we need to buy this converter, or this editing software, in order to deal with it."
"WHAT?!! That's outrageous! The software we have, even though it's 8 years old, should be able to edit ANYTHING!!"
"Well, it can't, I'm sorry."
"MAKE IT WORK!"
"Ok...well, I did. Sorta. But now it doesn't work. And I see that according to the software manual, it says I need to convert it to media it can edit."
"UGH! FINE! Here's money to get what you need."
Or
"Ugh, FINE! We just won't edit that then."
Or
"Ugh, FINE...you're fired!"
Actually, I don't know how it will go. But if it's video they are directing you to edit, you need to say "I need XX in order to do that." Hopefully they won't push back too much. But if they are still using really old software...I'm guessing they don't invest much in that area...