P.S. when you say focal extender I presume you mean Barlow??
No not really.
But if you already have the 42mm you might want to get the GSO 2" 2x barlow to go with it.
This will give you the equivalent of a 21mm 2" eyepiece but with an easy to use exit pupil distance ( the distance you eye needs to be from the eyepiece) .
You can also unscrew the barlow lens elements and screw them into the bottom of the 42mm EP and this will give you **about** a 28mm eyepiece.
So now you have 3 cheap eyepieces.
Myth #6: Barlow lenses degrade image quality.
There may have been some truth to this when Barlows were made with low-index glasses and not specifically designed for use with modern eyepieces. Modern, high-index Barlows actually improve eyepiece performance by reducing astigmatism at the edge of the field. Furthermore, using a Barlow increases the effective f/number of the objective and permits using longer-focal-length eyepieces (with their longer eye relief) for high-magnification viewing
What does this mean >>well your step light cone f5 now becomes an f10 and the light cone is now narrower and this allows the simpler design eyepiece to function better.
The principle behind Televue eyepieces!
They have built in barlows in the eyepiece design.
Don't expect your GSO to become a Televue