Yes, that's possible. But what is the purpose of Rockbox supporting more formats than mp3. Just so you don't have to convert before play.
I think you're the first person to ever ask about it. OMA is already a dead format, so you'll probably also be the last to ask about it too. That means its pretty much up to you to add support for it if you want to use it. Otherwise just use something normal like FLAC/WAV.
And if i convert to wav the file will be huge.
You said your file is PCM Hi-SP, which doesn't really make sense since they're different formats. If its really PCM, then it won't get any bigger since its already raw PCM which is as big as a file can get. If its Hi-SP, then it'll get bigger, but theres nothing you can do about it since Hi-SP is a proprietary format with no known specification or decoder source, so its impossible to play on anything but a few Sony devices IIRC.