No, they don't work that way.
Without extending the sub rail or replacing the rubber rails the simplest answer is this. Buy a set of cheap soft rubber pocket facings. Remove your hard facing (they are glued on) and put a soft one in its place. Trim to almost as small as the rail, but not quite. When the cloth is installed over it it will pull it in some, a trial and error deal. Or trim them thicker than the rail point so they have less give. Or a combination of the two. Then replace the hard facing over the soft one.
Rail cloth should be pulled tightly enough to see the stretch lines from the closely spaced staples for years after installation. Mines been about 8 years and you can still see them all.
Although I wouldn't recommended it for one pocket players, for other games it is a good alternative to pulling perfectly good rubber off the rails. I have personally never seen worn out rail rubber, even on tables approaching 100 years old, used 6 days a week since they were new.
I put 3 extra shims on mine to get 4" pockets. The sub rail not being extended didn't make any noticeable difference. But it's very hard with so many to carve them correctly.
Some tables have the cheap soft facings (olhausen?), in which case you can just get a hard facing and put it on the end.