Welcome to the forum.
If you use the SLICE command you can cut it in half (at the midpoint along the axis which you want to extend).
I like to use a CIRCLE as a slicing OBJECT, you could also define it by PLANE, once you start the SLICE command.
After you cut it in half switch to an ISOMETRIC view so that you can better see what you are doing.
Turn on ORTHO, then you can use PRESSPULL on the faces which you need to lengthen.
Only lengthen them by half of the total distance by which you want to stretch your frame.
Once you have the appropriate horizontal members lengthened you can switch to TOP view
and MIRROR your 3D solid about the center point (defined by the newly stretched horizontal members).
Go back into an ISOMETRIC perspective, or use ORBIT to confirm that everything looks good
before you use the UNION command to put it all back together with the new length.
You could also use the 3DMIRROR command directly from your isometric view without going back to
TOP view, but if you are not familiar with it, just using MIRROR from TOP view will do the same thing.