Paul,
If you look at a factory High Power barrel made before the middle 1990's, you will see that it is made in two pieces: the rear part containing the chamber, feed ramp, and the cam lock, and the front part containing the rifling and barrel locking lugs. The two parts are shrink fitted and silver soldered.
The short answer to your question is YES, a suitable .30 blank can be turned to fit a carefully bored-out old HP barrel. Use a low temperature silver fusion solder.
Finish chamber using a .30 Parabellum reamer and headspace to the slide.
The reason .30 Parabellum barrels are hard to find is that this caliber High Power was made for the European market, where countries such as Italy restricted civilian ownership of military caliber pistols.
HTH
John
