You can use an ETZ250 rim with a TS250 rear hub but better to use a the wider rim from a rear ETZ wheel as the front rim is really only wide enough for a 3.00″ tyre.
You will probably find that surgery is needed on the centre stand cross piece depending on size of tyre fitted. You will also probably find it overgeared with the standard 20t sprocket and need to drop to an 18t.
Another option is to find a complete ETZ250 rear wheel and sprocket carrier. This will fit staright in and can make use of your existing chain and chain rubbers. Saves having to build a special hibrid wheel.