It is easy to create a new custom unit. You will give this unit a certain set of dimensions and conversion information, and then you can use the unit anywhere a quantity with those dimensions is used.
First create the units with the right-click menu of the Units node in the Tools node:
This will result in a new blank unit:
You would then rename the unit
and then click the ... editor of the Dimensions property to set the dimensions. In this case the dimensions are length, so the length dimension exponent should be set to 1.
Press OK, and then set the Conversion and Description properties for the new unit:
You can now use this unit anywhere a quantity with dimensions of length is required.