I would say quite complicated. With OpenMRS only some of this can be done when using an auth provider and OAuth2. See this thread, amongst others. But only some level of users management will be possible, for instance roles management will continue to fully happen within OpenMRS.
But within Bahmni, again depending on what you hope to achieve exactly, this becomes up to three times more complicated since you have users in OpenMRS, OpenELIS and Odoo.
So I’d say
- OpenMRS only: quite complicated.
- Bahmni: very complicated.
A one time import of users into OpenMRS would require to write some kind of script that does it. It’s quite simple to import users with a SQL query for instance. Not 100% sure about the two others systems, probably the same level of complexity as with OpenMRS.