Basic questions. Is there a handle or something to be added to make my treadle (White Rotary from 1918) also be a handcrank?

No reproduction handcrank will work with a White FR. If you find an original you may be able to convert or add the handcrank. White made a Pearless sp? that was a handcrank which folded back.

I have used a steering wheel knob on mine. (Farm supply store) It took some adjusting. but unlike the Singer handcranks it doesn't change from 1:1 to 1:3. hand turns to machine turns. Makes for very slow stitches. Didn't use it long.