If it's a completely new soundfont (i.e. if there are no instruments in it), you have no choice but Ctrl+V for paste.
If there is at least one instrument, the right-click menu will work on that instrument.
Or creating a temporary empty instrument/preset can also be a solution.
Since there is no usual menu system in Polyphone 2.x versions (File, Edit, etc), some functions are only available in the right-click menu.
Developers can add the paste feature for the blank soundfont somewhere if they deem it necessary.