by my calculations, if a BM-21 rocket warhead had swing wings like a glide bomb, it would increase the range to 50-100km, but a guidance system would be needed. I wonder why we havent seen such things

In those glide bomb the bomb is strapped into the glide kit, the fins are not part of the bomb.

 

Where would you store the wings without altering the shape of the rocket? The fins need to be stowed in the area occupied by the rocket motor. Then you have the increased weight, unless you reduce the size of the warhead to compensate. And the higher drag from the wings. So lower peak velocity and altitude due to increased weight and higher drag, drop your range estimate by 40% to 50%

 

The reason most cruise missiles are jet powered is the wings are stowed into spaces in slots built into the fuel tanks

The other option is to incorporate the fin and guidance section between motor and warhead - it increases the overall length but many launchers are capable of absorbing that with limited problems - this is basically what the APKWS did. 

Those are mostly for guidance. He wants much larger ones for gliding.

With adequate materials a useful wing-area can be fitted with multiple folds into quite a compact area - or it can use the same type of aspect ratio + configuration as the JSOW for terminal glide at apex of trajectory when speed makes such a design efficient for the velocity in question - I'm not suggesting it's particularly advantageous or likely to be cheap but it's certainly not impossible.

 

R 

 

 

 

Lets say you take a BM-21 motor section and mount something like the Switchblade UAV on the front as the warhead section http://www.avinc.com/uas/adc/switchblade/
 

 You would separate the warhead/glide unit at apogee, ditching the burnt out rocket motor.

 

https://en.wikipedia.org/wiki/File:Katyusha_launcher_rear.jpg

https://en.wikipedia.org/wiki/File:Katyusha_launcher_rear.jpg  

Defeats the purpose in the boost phase.