How to assign business identifier without nesting

I have patient details with multiple hospital admissions and each hospital admission can have multiple ICU admissions.
I have mapped patient to patient resource,
hospital admissions to encounters
and ICU admissions to encounter to which is referenced to hospital encounters.

Now, how can i add hospital admission and ICU admission business identifier to ICU encounters.

Encounter supports 0…* identifier elements - so should be no issue with multiple identifiers from different sources on a single encounter.