Begin by reading the Data Types topic. Don't miss the discussion of VARCHAR and NVARCHAR types.
Is there a way using manifold 9 after file is import as a table to define for column an encoding and after change this encoding to UTF-8 ?
A table data type is either VARCHAR or NVARCHAR. NVARCHAR is Unicode using Manifold's internal storage to cover the entire Unicode standard. You don't define an encoding because all that is internal to Manifold: it covers the entire listing of all Unicode symbols.
UTF-8 is one of the three standard encodings (the others being UTF-16 and UTF-32) used in interchange formats to represent Unicode text, or are used by software that isn't fully Unicode as a way of handling Unicode characters.
Genuinely Unicode applications like Manifold are fairly rare. To be fully Unicode, you have to have full Unicode capability in everything that touches data, or you get into issues with encodings and such where some parts of the program correctly handle Unicode and others don't. What often happens is that programs write what they think is Unicode text but they don't get the nuances right.
A good example is Notepad, which will often display Unicode text within Notepad using the correct characters but then when it is saved as UTF-8 a genuinely Unicode program using that resulting text file will display different characters. Why? Because the text should have been saved as UTF-8 with BOM or UTF-16 or UTF-32, but not as UTF-8. "BOM" is a Byte Order Mark.
If you've imported something into a Manifold table and Unicode text is not showing up as it should, that's an indication that the format from which you imported didn't get the Unicode encoding right or didn't specify it. The classic case is importing "Unicode" from a text file, like CSV, that was saved using UTF-8 when UTF-8 with BOM should have been used. That's easy to fix: Open the text file in Notepad and then save it choosing UTF-8 with BOM in the Encoding box in the File - Save dialog.