If Cubase thinks you want to enter normal midi, then it's prob quite right to restrict the data to the range of 0 to 127 (although I wonder if there might be 'exceptions' to that rule).
But I would have expected Cubase to allow the entry of SYSEX, and that should allow up to FF, and include the values you want. BUT, is Cubase being 'clever' and/or 'helpful' and allowing SYSEX to be entered within the preset F0 and F7 bytes, or does it complain at a single byte SYSEX instruction? How does entering SYSEX work? Are there other options at that stage? Are you entering data into a midi file that is going to be 'played', or is there some way to SAVE and LOAD the midi data that you are trying to use, whereby the SAVEd file could be accessed and manually edited?