Combining Facet Values in Search and Promote

Question,

We are currently loading data into Search and Promote and creating Facets / Facet rails for specific metadata.

We have a situation where some indexed records contain metadata values pluralized or a 'like' term 

ie: (facet of 'type' has values of 'hat'; 'hats', 'ballcap') - and when creating the facet - we would like these all to display in the facet value selection as only 'hat'. 

ie: 

Type:
-- hat

-- hats (need to combine into hat and not display here)

-- ballcap (need to combine into hat and not display here)

Any assistance on accomplishing this?
(We even can 'rewrite' these values within the index first if that's possible - but unsure how to even do this...)

Thanks in advance!