It is unclear what you really want, can you please elaborate.
do you want to change the base unit of measure for material F1, (this is a frequntly asked question and not permitted again) or do you want to create the alternative units of measure in customizing?
piece is not a dimension.
Pieces are actually dimensionless. Any dimensionless unit has to created in dimension AAAADL dimension less. there is no big difference in creation of a dimensionless unit compared to a unit in a certain dimension. You just cannot define a SI unit and cannot have any auto conversion ratio between your new unit and the other units in the same dimension .
Once defined in CUNI, you enter the unit as alternative unit in your material master and exact here you define the conversion ratio between the materials base unit and this alternative unit.
And after that you can create your BoMs and enter the issuing unit individually