Hi Katy:
That’s right that the built-in Units table (exp.MeasurementUnits) can’t be expanded by users, since there is math among the scalable unit values like g->mg, mL->L, etc. You also wouldn’t want to redirect that built-in field’s lookup to your own list as it could create incompatibilities with other sample functionality.
One good way to accommodate this request is to add a new Text Choice field like “unitDescription” with any options your users want, like “block, slice, chunk, none”. The Sample Type would then be set to use “unit” as the units, the user would enter the number of blocks as # units (in the built in fields) and be able to select the type of unit in your other field. Of course if lots of Sample Types want this range of types, using a traditional lookup field to a shared list would also work well.
Hope that helps,
--Molly