I configured Lvl3 via editing /usr/share/X11/xkb/symbols/typo. But I found no documentation, how configure lvl5. I tried to add 2 more entries in this file, like this
key <AD05> { [NoSymbol, NoSymbol, trademark, registered, doubleacute] }; // "t"
But it seems to be error, as even lvl3 on this key ceased to work.