With exposed root bonsai it is possible to create a taller tree. One can simply raise the tree up on its finer root system while keeping it supported with wire until the new roots harden sufficiently to make the trunk stable.

The new roots will need to be covered for some months until they harden off and become adapted to dry air. Once so adapted the alluminum foil can be removed.

Ficus in exposed root style but a bit too short

Root system elongated,  wire used for temporary support

Foil is placed for some months until newly exposed roots are stable