How can I convert a regular radio into a shortwave radio?

Your idea is very good, at least this DIY spirit is very commendable!

The antenna is to be lengthened as you requested, but it's not as simple as wrapping a piece of copper wire around the antenna!

Get a radio board with shortwave reception and just plug into the radio!

1, the power supply should be appropriate, you can not take 12V into the 6V radio circuit, will burn out oh!

2, find the radio amplifier circuit, the short-wave audio signal injection for amplification.

3, consider, radio tuning, volume, band adjustment and antenna layout.

4, it is better to measure the size of the remodeling target and set your remodeling steps!

Believe in yourself, it's not hard, find multiple helpers or masters to work together! Wish you success! Haha