First, thank you for this service!

I have small audio database and want to use my database on the service. Is it possible?

Yes, that is possible. You can scan through your audio file and get associated metadata of that piece of audio.

Sorry, I don't understant that. E.G. I imported my all audio on MusicBrainz database, can I use it? And I don't know how can I use external database. Any documents for this option?

What type of service do you wish to use?
If you import audio files to your local device/machine and you want to access metadata of those audio, you should be fine. Please check out the MusicID-File section in the documentation. Thanks.