good, I found the way.
Meantime I can confirm that once installed OS2 Guest Additions onto OS2 virtual machine, then they will work in any host system.
The strings to input the custom screen resolution of the VDI machine were almost same of the ones given in the Windows cmd console, and.....
-it is essential that Virtualbox application is not running.
-no sudo mode should be activated in the Linux command terminal (and of course no Root as well)
-unlike in the Windows cmd case, no quotation marks must be used in the Linux command line.
this is how it worked for me:
mauro@mauro-HP:~$ VBoxManage setextradata global GUI/MaxGuestResolution any
mauro@mauro-HP:~$ VBoxManage setextradata OS2W452 CustomVideoMode1 1150x660x32
then I runned OS2 virtual machine and after system booted I went in the System Setup and found the screen resolution 1150x660x32 among the others, selected, closed the Setup window and rebooted; I found the new 1150x660 resolution working