I have a multiple questions :
Is that possible to convert a chord written into 1 voices to multiple voices if needed ?
How can I delete unwanted voices created before ?
The easiest way is to select the desired notes, and from the right-click context menu, Voices–Change voice.
If you have voices you want to remove, first you need to make sure no notes or explicit rests are assigned to that voice. Then when you close and re-open the project file, that voice will have been removed.
How can I enter chords using my midi keyboard and make Dorico interpret them as 2 voices or 3 voices and not as single notes ??? How can I make Dorico interpret my music as voices and not as single notes ?
You can’t enter multiple voices on a single staff simultaneously using the MIDI keyboard. That’s a tall order for a computer program! There are a handful of things you can do related to that, but not the exact thing you’re asking for.
Your best bet is to enter these notes one voice at a time. That gives you precise control over which notes are in which voices.