For an existing document right click at the corner where the rulers meet (upper left) and choose inches. It can also be done in Preferences/Units & Increments. If you want to be the default do it with no documents open.
You can insert in any field the default unit without any unit, but any other unit you can insert with the specific unit.
you dont need to convert it indesing can "translate" it him self an example 12 in + 12cm try to enter it i the size boxes