Is it possible to add custom tags in WMA files?

I want to add an "opus" tag (using foobar) to some of my tracks but it isn't working for my WMA's. Is it a restriction of the format or should I persist in trying to find some technical/system setup cause.

If I can't add a custom tag I could convert the tracks to another format or re-rip teh CDS but what format would you recommend - similar quality? similar size?

What settings do you use for encoding with WMA?

You can "invent" non-standard WMA metadata tag names, but it's probably not recommended. mp3tag can add any named tag to WMA files. But foobar2000 doesn't seem to allow it.

Here is a list of common WMA tags (as named internally): http://msdn2.microsoft.com/en-us/library/aa386866.aspx