How to build a walking closet
This might help you. https://www.hometalk.com/38263378/walking-closet-makeover?expand_all_questions=1
You would need to take up space from adjoining room to expand a closet.
You either have to bust into the room behind the existing closet or loose floor space in the room the closet is for.
Do you have a picture?
A picture and floorplan would be helpful!
If you can't cut back behind the closet, then go forward. Form a wooden frame and clad with timber, add a door or two and decorte to taste. Just a matter of how much room you need and how much room you have available for the task. Best wishes