I bought NXH-C110R tape measure type headphones, but the distance between my ears is too short to fit in my ears.

At the time of shipment, the left and right cords are left as one, and they are separated only near the main body, so please pull the cords by hand to the desired length before use.

Do you sell accessories such as earpieces and charging cables?

We do not sell only earpieces and charging cables.

Can it be used by inserting it into the earphone jack of a monaural radio?

Sound can only be output from one side.
Also, if you use a conversion plug, it is possible to output sound from both ears, but it will only be output in monaural sound.